[OpenTelemetry] Fix metricpoint race on ARM

[martincostello]

Changes

Fix flaky read on ARM that Claude code suggests is the root cause for test flakiness due to its weaker memory model in MetricPointReclaimTests found while working on #7400.

I can't validate this either way locally, so I told Claude to assume it was true and write a test to prove it in CI.

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 89.74%. Comparing base (6936f7e) to head (4d02376).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7401      +/-   ##
==========================================
+ Coverage   89.71%   89.74%   +0.03%     
==========================================
  Files         276      276              
  Lines       14576    14582       +6     
==========================================
+ Hits        13077    13087      +10     
+ Misses       1499     1495       -4     

Flag	Coverage Δ
unittests-Project-Experimental	89.64% <100.00%> (+0.14%)	⬆️
unittests-Project-Stable	89.65% <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
src/OpenTelemetry/Metrics/AggregatorStore.cs	89.38% <100.00%> (+1.06%)	⬆️
...c/OpenTelemetry/Metrics/MetricPoint/MetricPoint.cs	94.92% <100.00%> (+0.07%)	⬆️