[OpenTelemetry] Fix metric storage leak

[martincostello]

Fixes #5922

Changes

Fix metric storage leak that occurs when meters and instruments are repeatedly created and disposed.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.32%. Comparing base (fc89dbe) to head (7c54780).
⚠️ Report is 18 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7466      +/-   ##
==========================================
+ Coverage   90.22%   90.32%   +0.10%     
==========================================
  Files         287      287              
  Lines       15559    15798     +239     
==========================================
+ Hits        14038    14270     +232     
- Misses       1521     1528       +7     

Flag	Coverage Δ
unittests-Project-Experimental	90.14% <100.00%> (-0.11%)	⬇️
unittests-Project-Stable	90.19% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...rc/OpenTelemetry/Metrics/Reader/MetricReaderExt.cs	93.28% <100.00%> (+1.22%)	⬆️

... and 10 files with indirect coverage changes

[Copilot]

Pull request overview

This PR addresses a metrics storage memory leak in the OpenTelemetry SDK that occurs when Meter/instruments are repeatedly created and disposed (as in dotnet/runtime meter unpublish notifications). It does so by reclaiming freed metric storage slots and by removing deactivated instrument entries from the identity-to-metric dictionary so it cannot grow without bound.

Changes:

• Reclaim and reuse metric storage slots after deactivated metrics are removed during collection (availableMetricIndices + TryReserveMetricIndex).
• Prevent unbounded growth of instrumentIdentityToMetric by removing deactivated entries using value-matching removal to remain race-safe.
• Update/add unit tests validating storage reclamation and reuse across repeated collections; add a changelog entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/OpenTelemetry.Tests/Metrics/MeterProviderSdkTests.cs	Updates existing test expectations and adds a new regression test ensuring metric storage is reused across many transient meters/collects.
src/OpenTelemetry/Metrics/Reader/MetricReaderExt.cs	Implements index reuse for metric storage and removes deactivated metrics from the identity dictionary to eliminate leak vectors.
src/OpenTelemetry/CHANGELOG.md	Adds an Unreleased entry documenting the metric storage leak fix.

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[Kielek]

Changes LGTM, but it will be great if @alanwest or @cijothomas look into this also.