Add feature flag span enrichment system tests

[sameerank]

Motivation

Add system tests for feature flag event enrichment (Feature ID 551) that validates APM span chunk tags:

• ffe_flags_enc: base64(delta varint encoded serial IDs) for all evaluated flags
• ffe_subjects_enc: subject hash → serial IDs mapping (when doLog=true)
• ffe_defaults: flag name → "coded-default: " mapping (when flag not found in UFC)

Example span tags:

{
  "ffe_defaults": {
    "flag-not-found": "coded-default: my-default-value"
  },
  "ffe_flags_enc": "ZAgUAg==",
  "ffe_subjects_enc": {
    "4208f8d8017ce252df51f51e8c5b558a": "ZA==",
    "8e526f09f0531278f8bd60f224adf60a": "bAg="
  }
}

In this example: "ZAgUAg==" decodes to serial IDs [100, 108, 128, 130]

JIRA: https://datadoghq.atlassian.net/browse/FFL-2201

Changes

New test files (consolidated in parametric):

• tests/parametric/test_ffe/test_span_enrichment.py - All span enrichment tests
• tests/parametric/test_ffe/span-enrichment-flags.json - UFC fixture with serialId in splits
• tests/parametric/test_ffe/utils.py - Delta varint encoding/decoding utilities + SHA256 hashing

Test coverage:

Test Class	Test	Purpose
Test_Span_Enrichment_Serial_IDs	test_multiple_flags_serial_ids_combined	Multiple flags combine serial IDs correctly
Test_Span_Enrichment_Child_Span_Propagation	test_child_span_flag_evaluation_propagates_to_root	Child span propagates to root
Test_Span_Enrichment_Max_Serial_IDs	test_max_128_serial_ids_enforced	128 serial ID limit (generates 150 flags)
Test_Span_Enrichment_Default_Fallback	test_flag_not_found_adds_ffe_defaults_tag	Flag not in UFC adds coded-default prefix
Test_Span_Enrichment_Default_Fallback	test_ffe_defaults_value_truncated_at_64_chars	coded-default value truncated to 64 chars
Test_Span_Enrichment_Default_Fallback	test_max_5_flag_keys_in_ffe_defaults	Max 5 flag keys in ffe_defaults
Test_Span_Enrichment_Max_Subjects	test_max_25_subjects_enforced	25 subject limit
Test_Span_Enrichment_Subjects	test_do_log_true_adds_subjects_encoded	doLog=true adds subjects
Test_Span_Enrichment_Subjects	test_do_log_false_no_subjects_encoded	doLog=false skips subjects
Test_Span_Enrichment_Subjects	test_subjects_encoded_uses_sha256_hashed_targeting_key	Targeting keys are SHA256 hashed
Test_Span_Enrichment_Delta_Varint	5 encoding tests	Delta varint algorithm correctness

Manifest updates:

• Added missing_feature entries to language manifests (tests written before tracer implementation)

Why parametric only (no E2E)?

• Span enrichment is internal tracer behavior, not weblog behavior
• Weblog FFE integration is already tested by test_exposures.py and test_dynamic_evaluation.py
• Parametric tests provide precise control for edge cases (limits, child span propagation)

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

tests/parametric/test_ffe/span-enrichment-flags.json                    @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/parametric/test_ffe/test_span_enrichment.py                       @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/parametric/test_ffe/utils.py                                      @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
manifests/cpp.yml                                                       @DataDog/dd-trace-cpp
manifests/dotnet.yml                                                    @DataDog/apm-dotnet @DataDog/asm-dotnet
manifests/golang.yml                                                    @DataDog/dd-trace-go-guild
manifests/java.yml                                                      @DataDog/asm-java @DataDog/apm-java
manifests/nodejs.yml                                                    @DataDog/dd-trace-js
manifests/php.yml                                                       @DataDog/apm-php @DataDog/asm-php
manifests/python.yml                                                    @DataDog/apm-python @DataDog/asm-python
manifests/ruby.yml                                                      @DataDog/ruby-guild @DataDog/asm-ruby
manifests/rust.yml                                                      @DataDog/apm-rust
tests/parametric/test_ffe/test_dynamic_evaluation.py                    @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
utils/_features.py                                                      @DataDog/system-tests-core

[sameerank]

File name enforced by CI rule

system-tests/tests/test_the_test/test_conventions.py

Line 23 in 01d133a

if file in ("utils.py", "conftest.py", "__init__.py"):

[sameerank]

I just picked these numbers randomly, provisionally so I could write tests. What limits should we actually use?

[datadog-datadog-prod-us1-2]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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