feat(metrics): thread safety with concurrent collections and isolation for metrics

[hjgraca]

Please provide the issue number

Issue number: #1079

Summary

Changes

This pull request introduces thread-safe handling for metrics contexts, enabling correct behavior when Lambda functions process multiple concurrent requests (e.g., when AWS_LAMBDA_MAX_CONCURRENCY > 1). It also adds a new test project for concurrency testing and updates internal visibility accordingly.

Thread safety and concurrency improvements:

• Refactored Metrics in Metrics.cs to use a ConcurrentDictionary for per-thread metrics context storage. Shared configuration (namespace, service, default dimensions) is now kept in static fields, with thread-safe updates and propagation to all contexts. The singleton instance is now thread-safe, and all metrics operations (add, flush, set namespace/dimensions, etc.) operate on the current thread's context. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Other changes:

• Updated the XRayTraceId property in PowertoolsConfigurations.cs to use LambdaTraceProvider.CurrentTraceId for improved trace ID retrieval.
• Added missing using directives for required namespaces. [1] [2] [3]

User experience

Please share what the user experience looks like before and after this change

Checklist

Please leave checklist items unchecked if they do not apply to your change.

• Meets tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

• Migration process documented
• Implement warnings (if it can live side by side)

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Disclaimer: We value your time and bandwidth. As such, any pull requests created on non-triaged issues might not be successful.

[sonarqubecloud]

Quality Gate passed

Issues
30 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 86.25592% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.28%. Comparing base (eff1550) to head (cdbb572).
⚠️ Report is 21 commits behind head on develop.

Files with missing lines	Patch %	Lines
...Lambda.Powertools.Metrics/Model/MetricDirective.cs	79.41%	10 Missing and 4 partials ⚠️
...aries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs	90.55%	8 Missing and 4 partials ⚠️
...rc/AWS.Lambda.Powertools.Metrics/Model/Metadata.cs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1080      +/-   ##
===========================================
+ Coverage    78.18%   78.28%   +0.10%     
===========================================
  Files          298      298              
  Lines        12037    12122      +85     
  Branches      1449     1456       +7     
===========================================
+ Hits          9411     9490      +79     
- Misses        2161     2166       +5     
- Partials       465      466       +1     

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