feat: implement additional metric tags for trace stats

[duncanpharvey]

What does this PR do?

• Add DD_TRACE_STATS_ADDITIONAL_TAGS setting for adding additional tags to trace stats.
• Add DD_TRACE_STATS_ADDITIONAL_TAGS_CARDINALITY_LIMIT to limit number of distinct stats entries (default of 100 in libdd-trace-stats if no value is passed).
• Add DD_TRACE_EXPERIMENTAL_FEATURES_ENABLED to gate the behavior behind DD_TRACE_STATS_ADDITIONAL_TAGS and DD_TRACE_STATS_ADDITIONAL_TAGS_CARDINALITY_LIMIT.

Motivation

Allow Serverless Compatibility Layer to send additional metric tags on trace stats. See RFC.

https://datadoghq.atlassian.net/browse/SVLS-8787

Additional Notes

Dependent on these changes in libdatadog: DataDog/libdatadog#2170

Describe how to test/QA your changes

• Unit tests for regressions and new functionality added by DD_TRACE_EXPERIMENTAL_FEATURES_ENABLED
• Deploy to Azure Functions with the following environment variables and validate that additional tags appear on trace metrics
  • DD_TAGS: custom.primary:val,workload:azure_functions
  • DD_TRACE_EXPERIMENTAL_FEATURES_ENABLED: true
  • DD_TRACE_STATS_ADDITIONAL_TAGS: custom.primary,workload

[Copilot]

Pull request overview

Implements support for sending additional metric tags on trace stats by introducing new configuration settings (gated behind an experimental flag) and wiring them into the trace stats concentrator, along with updating the libdatadog dependency revision required for the feature.

Changes:

• Add DD_TRACE_EXPERIMENTAL_FEATURES_ENABLED, DD_TRACE_STATS_ADDITIONAL_TAGS, and DD_TRACE_STATS_ADDITIONAL_TAGS_CARDINALITY_LIMIT parsing to trace-agent config (with unit tests for gating).
• Update stats concentrator creation to pass additional metric tags + optional cardinality limit, and introduce a hashable key wrapper for TracerMetadata.
• Bump libdatadog git revs across crates and update lockfile / third-party license list accordingly.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
LICENSE-3rdparty.csv	Adds newly introduced transitive dependencies to the third-party license inventory.
crates/datadog-trace-agent/src/trace_processor.rs	Updates tests/struct construction to include newly added config fields and payload field.
crates/datadog-trace-agent/src/stats_concentrator_service.rs	Passes additional metric tags + cardinality limit into SpanConcentrator and updates metadata keying.
crates/datadog-trace-agent/src/config.rs	Adds experimental flag + additional tag settings, with unit tests for gating behavior.
crates/datadog-trace-agent/Cargo.toml	Bumps libdatadog git revision to pick up required upstream changes.
crates/datadog-serverless-compat/Cargo.toml	Bumps libdatadog git revision for trace utils.
crates/datadog-metrics-collector/Cargo.toml	Bumps libdatadog git revision for common utilities.
crates/datadog-agent-config/Cargo.toml	Bumps libdatadog git revision for trace utils/obfuscation.
Cargo.lock	Locks new upstream libdatadog revisions and new transitive crates introduced by the update.