[Infra/OpenTelemetry] Fix flaky test

[martincostello]

Fix flaky test observed in #6621.

Changes

Refactor MetricPointReclaimTests tests to avoid possible race condition, extend assertions and improve the code.

Will run a few times in draft to check for lack of failures.

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 86.82%. Comparing base (cb8d9e9) to head (b198f6d).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6622      +/-   ##
==========================================
+ Coverage   86.70%   86.82%   +0.11%     
==========================================
  Files         258      258              
  Lines       11955    11955              
==========================================
+ Hits        10366    10380      +14     
+ Misses       1589     1575      -14     

Flag	Coverage Δ
unittests-Project-Experimental	86.78% <ø> (+0.11%)	⬆️
unittests-Project-Stable	86.76% <ø> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 5 files with indirect coverage changes

[Copilot]

Pull Request Overview

This PR refactors MetricPointReclaimTests to fix a flaky test by addressing potential race conditions and improving code quality. The changes focus on thread-safety improvements and code modernization.

Key Changes:

• Added thread-safe sum accumulation using Interlocked.Add to prevent race conditions
• Added assertions on ForceFlush() return values to verify export success
• Modernized code syntax (collection expressions, simplified conditionals)

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[martincostello]

I'm not 100% sure these changes definitely fix the issue observed, but there's been no reoccurrence in 5 re-runs.