fix: concurrency issues in Metrics

[hjgraca]

Please provide the issue number

Issue number: #1000

Summary

Fixes concurrency issues in the Metrics utility that were causing IndexOutOfRangeException and InvalidOperationException in multi-threaded environments.

Issues Fixed

• Collection modification during enumeration: LINQ FirstOrDefault operations were failing when collections were modified concurrently
• Dictionary thread safety: CustomMetadata dictionary was not thread-safe, causing IndexOutOfRangeException during concurrent access

Changes Made

Core Fixes

• Replaced LINQ enumeration with thread-safe iteration: Added GetExistingMetric helper methods in Metrics.cs and MetricDirective.cs that use traditional for-loops instead of FirstOrDefault
• Implemented thread-safe metadata storage: Changed CustomMetadata from Dictionary<string, object> to ConcurrentDictionary<string, object> in Metadata.cs
• Added exception handling: Graceful handling of ArgumentOutOfRangeException during concurrent collection modifications

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

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[codecov]

Codecov Report

❌ Patch coverage is 79.77528% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.80%. Comparing base (4c3c3ce) to head (6742670).
⚠️ Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
...Lambda.Powertools.Metrics/Model/MetricDirective.cs	75.00%	21 Missing and 11 partials ⚠️
...aries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs	68.57%	8 Missing and 3 partials ⚠️
...rc/AWS.Lambda.Powertools.Metrics/Model/RootNode.cs	86.07%	0 Missing and 11 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## develop #1001 +/- ## =========================================== - Coverage 77.89% 77.80% -0.09%  =========================================== Files 287 287 Lines 11444 11647 +203 Branches 1346 1391 +45 =========================================== + Hits 8914 9062 +148  - Misses 2099 2133 +34  - Partials 431 452 +21 

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[sonarqubecloud]

Quality Gate passed

Issues
0 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