RUM-16535: Add APM client side stats models and encoder

[abrooksv]

What does this PR do?

Models the data classes for the APM client side stats payloads: (definition)

Note that while that definition is for a protobuf format, we will be uploading them in their MsgPack format, and the field mappings can be found here

To keep runtime dependency graph small, a minimal encoder is included, but a test dependency on a reference implementation is included in order to validate the implementation

Motivation

This PR models the classes that will eventually be uploaded to the stats intake endpoint.

Additional Notes

The fields we will be sending are based off of the iOS RFC, but which has not yet been put out for code review

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (feature/client-side-stats@59852c8). Learn more about missing BASE report.

Additional details and impacted files

@@                     Coverage Diff                      @@
##             feature/client-side-stats    #3487   +/-   ##
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  Coverage                             ?   72.44%           
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  Files                                ?      967           
  Lines                                ?    35834           
  Branches                             ?     5951           
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  Hits                                 ?    25958           
  Misses                               ?     8269           
  Partials                             ?     1607           

Files with missing lines	Coverage Δ
...race/internal/domain/metrics/ClientStatsPayload.kt	100.00% <100.00%> (ø)
...id/trace/internal/domain/metrics/MsgPackEncoder.kt	100.00% <100.00%> (ø)
...roid/trace/internal/domain/metrics/StatsPayload.kt	100.00% <100.00%> (ø)

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[Copilot]

Pull request overview

This PR introduces the client-side APM stats payload model types for the trace feature and a minimal MsgPack encoder to serialize them (with tests validating the produced payload structure against a reference MsgPack unpacker).

Changes:

• Added stats payload models (StatsPayload, ClientStatsPayload, buckets, grouped stats, and Trilean) with toMsgPackPayload() serialization.
• Implemented a minimal MsgPackEncoder and accompanying tests to validate encoding of core MsgPack primitives and headers.
• Updated build/test configuration to include msgpack-core, and updated 3rd-party licensing + detekt safe-call allowlist.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
LICENSE-3rdparty.csv	Adds 3rd-party license entry for org.msgpack:msgpack-core (test dependency).
gradle/libs.versions.toml	Adds version catalog entry for msgPackCore.
features/dd-sdk-android-trace/build.gradle.kts	Adds msgpack-core as a test dependency for the trace feature.
detekt_custom_safe_calls.yml	Allows additional safe calls used by the new encoder implementation.
features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/domain/metrics/StatsPayload.kt	Adds outer envelope model and MsgPack encoding for the stats intake payload.
features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/domain/metrics/MsgPackEncoder.kt	Adds the minimal MsgPack encoder used to build payload bytes.
features/dd-sdk-android-trace/src/main/kotlin/com/datadog/android/trace/internal/domain/metrics/ClientStatsPayload.kt	Adds client stats payload/bucket/grouped stats models and encoding.
features/dd-sdk-android-trace/src/test/kotlin/com/datadog/android/trace/internal/domain/metrics/StatsPayloadTest.kt	Validates the outer payload fields and embedding of pre-encoded client stats.
features/dd-sdk-android-trace/src/test/kotlin/com/datadog/android/trace/internal/domain/metrics/MsgPackEncoderTest.kt	Validates encoding of MsgPack primitives/headers via msgpack-core unpacking.
features/dd-sdk-android-trace/src/test/kotlin/com/datadog/android/trace/internal/domain/metrics/ClientStatsPayloadTest.kt	Validates the client stats payload field mapping and nested objects.
features/dd-sdk-android-trace/src/test/kotlin/com/datadog/android/trace/assertj/MsgPackAssert.kt	Adds a MsgPack-to-JSON assert helper for payload structure assertions.

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[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.