From 7658d022c145b9f1a348235c10af4e87bb86a027 Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 14:53:10 -0700
Subject: [PATCH 1/7] attempt deduplicating x-goog-api-client headers

---
 .../google/api_core/gapic_v1/method.py        | 27 +++++++++++++------
 .../google/api_core/grpc_helpers.py           | 19 +++++++++----
 .../google-auth/google/auth/transport/grpc.py |  6 ++++-
 .../google-auth/tests/transport/test_grpc.py  | 27 +++++++++++++++++++
 4 files changed, 65 insertions(+), 14 deletions(-)

diff --git a/packages/google-api-core/google/api_core/gapic_v1/method.py b/packages/google-api-core/google/api_core/gapic_v1/method.py
index b4481ca198a4..f4cc3eaca471 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/method.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/method.py
@@ -114,14 +114,25 @@ def __call__(
 
         # Add the user agent metadata to the call.
         if self._metadata is not None:
-            metadata = kwargs.get("metadata", [])
-            # Due to the nature of invocation, None should be treated the same
-            # as not specified.
-            if metadata is None:
-                metadata = []
-            metadata = list(metadata)
-            metadata.extend(self._metadata)
-            kwargs["metadata"] = metadata
+            metadata = kwargs.get("metadata")
+            if not metadata:
+                kwargs["metadata"] = self._metadata
+            else:
+                # Merge user-supplied metadata with library-supplied metadata.
+                # All keys in gRPC metadata are already lowercase.
+                from itertools import chain
+                metadata = list(metadata)
+                api_client_values = []
+                merged_metadata = []
+                for key, val in chain(metadata, self._metadata):
+                    if key == "x-goog-api-client":
+                        api_client_values.append(val)
+                    else:
+                        merged_metadata.append((key, val))
+                if api_client_values:
+                    merged_metadata.append(("x-goog-api-client", " ".join(api_client_values)))
+                kwargs["metadata"] = merged_metadata
+
         if self._compression is not None:
             kwargs["compression"] = compression
 
diff --git a/packages/google-api-core/google/api_core/grpc_helpers.py b/packages/google-api-core/google/api_core/grpc_helpers.py
index 30ba19c54f1a..01b9092a2a26 100644
--- a/packages/google-api-core/google/api_core/grpc_helpers.py
+++ b/packages/google-api-core/google/api_core/grpc_helpers.py
@@ -254,11 +254,20 @@ def _create_composite_credentials(
     request = google.auth.transport.requests.Request()
 
     # Create the metadata plugin for inserting the authorization header.
-    metadata_plugin = google.auth.transport.grpc.AuthMetadataPlugin(
-        credentials,
-        request,
-        default_host=default_host,
-    )
+    try:
+        metadata_plugin = google.auth.transport.grpc.AuthMetadataPlugin(
+            credentials,
+            request,
+            default_host=default_host,
+            suppress_metrics_header=True,
+        )
+    except TypeError:
+        # Support older versions of google-auth that do not accept suppress_metrics_header
+        metadata_plugin = google.auth.transport.grpc.AuthMetadataPlugin(
+            credentials,
+            request,
+            default_host=default_host,
+        )
 
     # Create a set of grpc.CallCredentials using the metadata plugin.
     google_auth_credentials = grpc.metadata_call_credentials(metadata_plugin)
diff --git a/packages/google-auth/google/auth/transport/grpc.py b/packages/google-auth/google/auth/transport/grpc.py
index e541d20ca0a4..3952bbeae63b 100644
--- a/packages/google-auth/google/auth/transport/grpc.py
+++ b/packages/google-auth/google/auth/transport/grpc.py
@@ -49,7 +49,7 @@ class AuthMetadataPlugin(grpc.AuthMetadataPlugin):
             account credentials.
     """
 
-    def __init__(self, credentials, request, default_host=None):
+    def __init__(self, credentials, request, default_host=None, suppress_metrics_header=False):
         # pylint: disable=no-value-for-parameter
         # pylint doesn't realize that the super method takes no arguments
         # because this class is the same name as the superclass.
@@ -57,6 +57,7 @@ def __init__(self, credentials, request, default_host=None):
         self._credentials = credentials
         self._request = request
         self._default_host = default_host
+        self._suppress_metrics_header = suppress_metrics_header
 
     def _get_authorization_headers(self, context):
         """Gets the authorization headers for a request.
@@ -80,6 +81,9 @@ def _get_authorization_headers(self, context):
             self._request, context.method_name, context.service_url, headers
         )
 
+        if self._suppress_metrics_header and "x-goog-api-client" in headers:
+            del headers["x-goog-api-client"]
+
         return list(headers.items())
 
     def __call__(self, context, callback):
diff --git a/packages/google-auth/tests/transport/test_grpc.py b/packages/google-auth/tests/transport/test_grpc.py
index 7ebd14758e55..6907d3be8b3c 100644
--- a/packages/google-auth/tests/transport/test_grpc.py
+++ b/packages/google-auth/tests/transport/test_grpc.py
@@ -132,6 +132,33 @@ def test__get_authorization_headers_with_service_account_and_default_host(self):
             "https://{}/".format(default_host)
         )
 
+    def test_suppress_metrics_header(self):
+        credentials = mock.create_autospec(service_account.Credentials)
+        # Mock credentials before_request that adds metric and authorization
+        def mock_before_request(request, method, url, headers):
+            headers["x-goog-api-client"] = "foo"
+            headers["authorization"] = "Bearer token"
+        credentials.before_request.side_effect = mock_before_request
+        request = mock.create_autospec(transport.Request)
+
+        # By default, suppress_metrics_header=False
+        plugin = google.auth.transport.grpc.AuthMetadataPlugin(credentials, request)
+        context = mock.create_autospec(grpc.AuthMetadataContext, instance=True)
+        context.method_name = "methodName"
+        context.service_url = "https://pubsub.googleapis.com/methodName"
+
+        headers = dict(plugin._get_authorization_headers(context))
+        assert "x-goog-api-client" in headers
+        assert headers["x-goog-api-client"] == "foo"
+
+        # With suppress_metrics_header=True
+        plugin_suppressed = google.auth.transport.grpc.AuthMetadataPlugin(
+            credentials, request, suppress_metrics_header=True
+        )
+        headers_suppressed = dict(plugin_suppressed._get_authorization_headers(context))
+        assert "x-goog-api-client" not in headers_suppressed
+        assert headers_suppressed["authorization"] == "Bearer token"
+
 
 @mock.patch(
     "google.auth.transport._mtls_helper.get_client_ssl_credentials", autospec=True

From 7d266a352bb1e188f79f2eaf3397e9d824104701 Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 15:27:36 -0700
Subject: [PATCH 2/7] added test

---
 .../tests/unit/gapic/test_method.py           | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/packages/google-api-core/tests/unit/gapic/test_method.py b/packages/google-api-core/tests/unit/gapic/test_method.py
index 29e8fc2173bd..a6e7f03e7b0d 100644
--- a/packages/google-api-core/tests/unit/gapic/test_method.py
+++ b/packages/google-api-core/tests/unit/gapic/test_method.py
@@ -121,6 +121,39 @@ def test_invoke_wrapped_method_with_metadata_as_none():
     assert len(metadata) == 1
 
 
+def test_invoke_wrapped_method_with_duplicate_x_goog_api_client_metadata():
+    method = mock.Mock(spec=["__call__"])
+
+    # Create a custom ClientInfo with defined properties so we know exactly what is returned
+    client_info = google.api_core.gapic_v1.client_info.ClientInfo(
+        user_agent="custom-user-agent/1.0",
+        python_version="3.14.0",
+        grpc_version="1.76.0",
+        api_core_version="2.29.0"
+    )
+
+    wrapped_method = google.api_core.gapic_v1.method.wrap_method(method, client_info=client_info)
+
+    # Invoke the wrapped method with an explicit user-provided custom header
+    wrapped_method(
+        mock.sentinel.request,
+        metadata=[("x-goog-api-client", "override-client/2.0"), ("other-header", "value")]
+    )
+
+    method.assert_called_once_with(mock.sentinel.request, metadata=mock.ANY)
+    metadata = method.call_args[1]["metadata"]
+
+    # There should only be one "x-goog-api-client" header, containing both values joined by space,
+    # plus the other-header.
+    assert len(metadata) == 2
+    metadata_dict = dict(metadata)
+    assert "other-header" in metadata_dict
+    assert metadata_dict["other-header"] == "value"
+    assert "x-goog-api-client" in metadata_dict
+    # Verify both the user-provided override value and the library system telemetry are merged explicitly
+    assert metadata_dict["x-goog-api-client"] == "override-client/2.0 custom-user-agent/1.0 gl-python/3.14.0 grpc/1.76.0 gax/2.29.0" 
+
+
 @mock.patch("time.sleep")
 def test_wrap_method_with_default_retry_and_timeout_and_compression(unused_sleep):
     method = mock.Mock(

From 852c5b1d6e073f8da3f1c06ce0cc3b66a380a390 Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 15:27:52 -0700
Subject: [PATCH 3/7] optimized for hard-coded metadata

---
 .../google/api_core/gapic_v1/method.py        | 27 ++++++++++++-------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/packages/google-api-core/google/api_core/gapic_v1/method.py b/packages/google-api-core/google/api_core/gapic_v1/method.py
index f4cc3eaca471..49fa7f591f15 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/method.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/method.py
@@ -91,6 +91,14 @@ def __init__(
         self._timeout = timeout
         self._compression = compression
         self._metadata = metadata
+        # separate x-goog-api-client header from provided metadata
+        self._arbitrary_metadata = []
+        self._metrics_values = ""
+        for key, val in metadata:
+            if key == client_info.METRICS_METADATA_KEY:
+                self._metrics_values = val
+            else:
+                self._arbitrary_metadata.append((key, val))
 
     def __call__(
         self, *args, timeout=DEFAULT, retry=DEFAULT, compression=DEFAULT, **kwargs
@@ -116,21 +124,22 @@ def __call__(
         if self._metadata is not None:
             metadata = kwargs.get("metadata")
             if not metadata:
-                kwargs["metadata"] = self._metadata
+                if self._metrics_values:
+                    kwargs["metadata"] = [(client_info.METRICS_METADATA_KEY, self._metrics_values), *self._arbitrary_metadata]
+                else:
+                    kwargs["metadata"] = self._arbitrary_metadata
             else:
                 # Merge user-supplied metadata with library-supplied metadata.
-                # All keys in gRPC metadata are already lowercase.
-                from itertools import chain
                 metadata = list(metadata)
-                api_client_values = []
+                metric_values = [self._metrics_values] if self._metrics_values else []
                 merged_metadata = []
-                for key, val in chain(metadata, self._metadata):
-                    if key == "x-goog-api-client":
-                        api_client_values.append(val)
+                for key, val in metadata:
+                    if key == client_info.METRICS_METADATA_KEY:
+                        metric_values.append(val)
                     else:
                         merged_metadata.append((key, val))
-                if api_client_values:
-                    merged_metadata.append(("x-goog-api-client", " ".join(api_client_values)))
+                if metric_values:
+                    merged_metadata.append((client_info.METRICS_METADATA_KEY, " ".join(metric_values)))
                 kwargs["metadata"] = merged_metadata
 
         if self._compression is not None:

From 0d8865a1dda0df0ba6e9c6f3de00415ef0134f18 Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 15:34:29 -0700
Subject: [PATCH 4/7] cleaning up code

---
 .../google/api_core/gapic_v1/method.py        | 31 ++++++++++++-------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/packages/google-api-core/google/api_core/gapic_v1/method.py b/packages/google-api-core/google/api_core/gapic_v1/method.py
index 49fa7f591f15..486dbb3a478f 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/method.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/method.py
@@ -91,14 +91,17 @@ def __init__(
         self._timeout = timeout
         self._compression = compression
         self._metadata = metadata
-        # separate x-goog-api-client header from provided metadata
+        
+        # Pre-extract the client metrics header from the initialized metadata.
+        # This avoids repeating this work on every single RPC request invocation.
         self._arbitrary_metadata = []
         self._metrics_values = ""
-        for key, val in metadata:
-            if key == client_info.METRICS_METADATA_KEY:
-                self._metrics_values = val
-            else:
-                self._arbitrary_metadata.append((key, val))
+        if metadata:
+            for key, val in metadata:
+                if key == client_info.METRICS_METADATA_KEY:
+                    self._metrics_values = val
+                else:
+                    self._arbitrary_metadata.append((key, val))
 
     def __call__(
         self, *args, timeout=DEFAULT, retry=DEFAULT, compression=DEFAULT, **kwargs
@@ -124,22 +127,28 @@ def __call__(
         if self._metadata is not None:
             metadata = kwargs.get("metadata")
             if not metadata:
+                # Fast path: in 99% of calls, the user did not pass any custom metadata,
+                # so we can directly assign the pre-extracted metadata and skip any merging overhead.
                 if self._metrics_values:
-                    kwargs["metadata"] = [(client_info.METRICS_METADATA_KEY, self._metrics_values), *self._arbitrary_metadata]
+                    kwargs["metadata"] = [(client_info.METRICS_METADATA_KEY, self._metrics_values)] + self._arbitrary_metadata
                 else:
                     kwargs["metadata"] = self._arbitrary_metadata
             else:
                 # Merge user-supplied metadata with library-supplied metadata.
+                # All keys in gRPC metadata are already lowercase.
                 metadata = list(metadata)
-                metric_values = [self._metrics_values] if self._metrics_values else []
+                api_client_values = []
                 merged_metadata = []
                 for key, val in metadata:
                     if key == client_info.METRICS_METADATA_KEY:
-                        metric_values.append(val)
+                        api_client_values.append(val)
                     else:
                         merged_metadata.append((key, val))
-                if metric_values:
-                    merged_metadata.append((client_info.METRICS_METADATA_KEY, " ".join(metric_values)))
+                if self._metrics_values:
+                    api_client_values.append(self._metrics_values)
+                if api_client_values:
+                    merged_metadata.append((client_info.METRICS_METADATA_KEY, " ".join(api_client_values)))
+                merged_metadata.extend(self._arbitrary_metadata)
                 kwargs["metadata"] = merged_metadata
 
         if self._compression is not None:

From 76274cab6d73b01a27c54b27e3f052808f0f743e Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 16:33:08 -0700
Subject: [PATCH 5/7] pulled out helper method

---
 .../google/api_core/gapic_v1/method.py        | 54 +++++++++++--------
 1 file changed, 32 insertions(+), 22 deletions(-)

diff --git a/packages/google-api-core/google/api_core/gapic_v1/method.py b/packages/google-api-core/google/api_core/gapic_v1/method.py
index 486dbb3a478f..278dad9cf51c 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/method.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/method.py
@@ -57,6 +57,34 @@ def _apply_decorators(func, decorators):
     return func
 
 
+def _extract_metrics_header(metadata):
+    """Extract x-google-api-client header from metadata list.
+
+    Args:
+        metadata (Sequence[Tuple[str, str]]): The metadata to extract from.
+
+    Returns:
+        Tuple[List[Tuple[str, str]], List[str]]: A tuple containing:
+            - A list of remaining metadata tuples.
+            - A list of metrics header values found.
+    """
+    if not metadata:
+        return [], []
+
+    for i, (key, val) in enumerate(metadata):
+        if key == client_info.METRICS_METADATA_KEY:
+            # Key located. Check the rest of the list for duplicate entries
+            arbitrary_metadata = list(metadata[:i])
+            metric_values = [val]
+            for k, v in metadata[i+1:]:
+                if k == client_info.METRICS_METADATA_KEY:
+                    metric_values.append(v)
+                else:
+                    arbitrary_metadata.append((k, v))
+            return arbitrary_metadata, metric_values
+    # No key found
+    return list(metadata), []
+
 class _GapicCallable(object):
     """Callable that applies retry, timeout, and metadata logic.
 
@@ -91,17 +119,9 @@ def __init__(
         self._timeout = timeout
         self._compression = compression
         self._metadata = metadata
-        
-        # Pre-extract the client metrics header from the initialized metadata.
-        # This avoids repeating this work on every single RPC request invocation.
-        self._arbitrary_metadata = []
-        self._metrics_values = ""
-        if metadata:
-            for key, val in metadata:
-                if key == client_info.METRICS_METADATA_KEY:
-                    self._metrics_values = val
-                else:
-                    self._arbitrary_metadata.append((key, val))
+        # Pre-extract the x-goog-api-client header from the initialized metadata.
+        self._arbitrary_metadata, metric_values = _extract_metrics_header(metadata)
+        self._metrics_values = " ".join(metric_values) if metric_values else ""
 
     def __call__(
         self, *args, timeout=DEFAULT, retry=DEFAULT, compression=DEFAULT, **kwargs
@@ -127,23 +147,13 @@ def __call__(
         if self._metadata is not None:
             metadata = kwargs.get("metadata")
             if not metadata:
-                # Fast path: in 99% of calls, the user did not pass any custom metadata,
-                # so we can directly assign the pre-extracted metadata and skip any merging overhead.
                 if self._metrics_values:
                     kwargs["metadata"] = [(client_info.METRICS_METADATA_KEY, self._metrics_values)] + self._arbitrary_metadata
                 else:
                     kwargs["metadata"] = self._arbitrary_metadata
             else:
                 # Merge user-supplied metadata with library-supplied metadata.
-                # All keys in gRPC metadata are already lowercase.
-                metadata = list(metadata)
-                api_client_values = []
-                merged_metadata = []
-                for key, val in metadata:
-                    if key == client_info.METRICS_METADATA_KEY:
-                        api_client_values.append(val)
-                    else:
-                        merged_metadata.append((key, val))
+                merged_metadata, api_client_values = _extract_metrics_header(metadata)
                 if self._metrics_values:
                     api_client_values.append(self._metrics_values)
                 if api_client_values:

From e42f0a3d8359b077abaa6a44ad1c645cb46de9d5 Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 16:49:02 -0700
Subject: [PATCH 6/7] added docstring

---
 packages/google-auth/google/auth/transport/grpc.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/packages/google-auth/google/auth/transport/grpc.py b/packages/google-auth/google/auth/transport/grpc.py
index 3952bbeae63b..93f392731163 100644
--- a/packages/google-auth/google/auth/transport/grpc.py
+++ b/packages/google-auth/google/auth/transport/grpc.py
@@ -47,9 +47,11 @@ class AuthMetadataPlugin(grpc.AuthMetadataPlugin):
         default_host (Optional[str]): A host like "pubsub.googleapis.com".
             This is used when a self-signed JWT is created from service
             account credentials.
+        suppress_metrics_header (bool): When enabled, ``x-goog-api-client``
+            will be stripped from authorization headers.
     """
 
-    def __init__(self, credentials, request, default_host=None, suppress_metrics_header=False):
+    def __init__(self, credentials, request, default_host=None, *, suppress_metrics_header=False):
         # pylint: disable=no-value-for-parameter
         # pylint doesn't realize that the super method takes no arguments
         # because this class is the same name as the superclass.

From fc05e08dd21f0f7e86d444a7f93905c0450fccd6 Mon Sep 17 00:00:00 2001
From: Daniel Sanche <d.sanche14@gmail.com>
Date: Wed, 1 Jul 2026 17:04:42 -0700
Subject: [PATCH 7/7] fixed format

---
 .../google/api_core/gapic_v1/method.py           | 11 ++++++++---
 .../tests/unit/gapic/test_method.py              | 16 ++++++++++++----
 .../google-auth/google/auth/transport/grpc.py    |  4 +++-
 .../google-auth/tests/transport/test_grpc.py     |  2 ++
 4 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/packages/google-api-core/google/api_core/gapic_v1/method.py b/packages/google-api-core/google/api_core/gapic_v1/method.py
index 278dad9cf51c..b56463cf1770 100644
--- a/packages/google-api-core/google/api_core/gapic_v1/method.py
+++ b/packages/google-api-core/google/api_core/gapic_v1/method.py
@@ -76,7 +76,7 @@ def _extract_metrics_header(metadata):
             # Key located. Check the rest of the list for duplicate entries
             arbitrary_metadata = list(metadata[:i])
             metric_values = [val]
-            for k, v in metadata[i+1:]:
+            for k, v in metadata[i + 1 :]:
                 if k == client_info.METRICS_METADATA_KEY:
                     metric_values.append(v)
                 else:
@@ -85,6 +85,7 @@ def _extract_metrics_header(metadata):
     # No key found
     return list(metadata), []
 
+
 class _GapicCallable(object):
     """Callable that applies retry, timeout, and metadata logic.
 
@@ -148,7 +149,9 @@ def __call__(
             metadata = kwargs.get("metadata")
             if not metadata:
                 if self._metrics_values:
-                    kwargs["metadata"] = [(client_info.METRICS_METADATA_KEY, self._metrics_values)] + self._arbitrary_metadata
+                    kwargs["metadata"] = [
+                        (client_info.METRICS_METADATA_KEY, self._metrics_values)
+                    ] + self._arbitrary_metadata
                 else:
                     kwargs["metadata"] = self._arbitrary_metadata
             else:
@@ -157,7 +160,9 @@ def __call__(
                 if self._metrics_values:
                     api_client_values.append(self._metrics_values)
                 if api_client_values:
-                    merged_metadata.append((client_info.METRICS_METADATA_KEY, " ".join(api_client_values)))
+                    merged_metadata.append(
+                        (client_info.METRICS_METADATA_KEY, " ".join(api_client_values))
+                    )
                 merged_metadata.extend(self._arbitrary_metadata)
                 kwargs["metadata"] = merged_metadata
 
diff --git a/packages/google-api-core/tests/unit/gapic/test_method.py b/packages/google-api-core/tests/unit/gapic/test_method.py
index a6e7f03e7b0d..337451e0ed83 100644
--- a/packages/google-api-core/tests/unit/gapic/test_method.py
+++ b/packages/google-api-core/tests/unit/gapic/test_method.py
@@ -129,15 +129,20 @@ def test_invoke_wrapped_method_with_duplicate_x_goog_api_client_metadata():
         user_agent="custom-user-agent/1.0",
         python_version="3.14.0",
         grpc_version="1.76.0",
-        api_core_version="2.29.0"
+        api_core_version="2.29.0",
     )
 
-    wrapped_method = google.api_core.gapic_v1.method.wrap_method(method, client_info=client_info)
+    wrapped_method = google.api_core.gapic_v1.method.wrap_method(
+        method, client_info=client_info
+    )
 
     # Invoke the wrapped method with an explicit user-provided custom header
     wrapped_method(
         mock.sentinel.request,
-        metadata=[("x-goog-api-client", "override-client/2.0"), ("other-header", "value")]
+        metadata=[
+            ("x-goog-api-client", "override-client/2.0"),
+            ("other-header", "value"),
+        ],
     )
 
     method.assert_called_once_with(mock.sentinel.request, metadata=mock.ANY)
@@ -151,7 +156,10 @@ def test_invoke_wrapped_method_with_duplicate_x_goog_api_client_metadata():
     assert metadata_dict["other-header"] == "value"
     assert "x-goog-api-client" in metadata_dict
     # Verify both the user-provided override value and the library system telemetry are merged explicitly
-    assert metadata_dict["x-goog-api-client"] == "override-client/2.0 custom-user-agent/1.0 gl-python/3.14.0 grpc/1.76.0 gax/2.29.0" 
+    assert (
+        metadata_dict["x-goog-api-client"]
+        == "override-client/2.0 custom-user-agent/1.0 gl-python/3.14.0 grpc/1.76.0 gax/2.29.0"
+    )
 
 
 @mock.patch("time.sleep")
diff --git a/packages/google-auth/google/auth/transport/grpc.py b/packages/google-auth/google/auth/transport/grpc.py
index 93f392731163..df3f8a7dcf54 100644
--- a/packages/google-auth/google/auth/transport/grpc.py
+++ b/packages/google-auth/google/auth/transport/grpc.py
@@ -51,7 +51,9 @@ class AuthMetadataPlugin(grpc.AuthMetadataPlugin):
             will be stripped from authorization headers.
     """
 
-    def __init__(self, credentials, request, default_host=None, *, suppress_metrics_header=False):
+    def __init__(
+        self, credentials, request, default_host=None, *, suppress_metrics_header=False
+    ):
         # pylint: disable=no-value-for-parameter
         # pylint doesn't realize that the super method takes no arguments
         # because this class is the same name as the superclass.
diff --git a/packages/google-auth/tests/transport/test_grpc.py b/packages/google-auth/tests/transport/test_grpc.py
index 6907d3be8b3c..15f7e6b62842 100644
--- a/packages/google-auth/tests/transport/test_grpc.py
+++ b/packages/google-auth/tests/transport/test_grpc.py
@@ -134,10 +134,12 @@ def test__get_authorization_headers_with_service_account_and_default_host(self):
 
     def test_suppress_metrics_header(self):
         credentials = mock.create_autospec(service_account.Credentials)
+
         # Mock credentials before_request that adds metric and authorization
         def mock_before_request(request, method, url, headers):
             headers["x-goog-api-client"] = "foo"
             headers["authorization"] = "Bearer token"
+
         credentials.before_request.side_effect = mock_before_request
         request = mock.create_autospec(transport.Request)