[SDK] Replace SpinLockMutex with std::mutex in the metrics library

[dbarker]

Fixes #4317

Part 2 of #4317 to replace SpinLockMutex with std::mutex.

This PR replaces most use of the SpinLockMutex in the metrics sdk. The goal is to improve synchronization consistency across platforms and deployments by relying on the standard mutex.

Changes

• Replaces SpinLockMutex with std::mutex in the metrics storage classes.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.23%. Comparing base (d3b1670) to head (a5e0b88).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4416      +/-   ##
==========================================
+ Coverage   83.22%   83.23%   +0.01%     
==========================================
  Files         520      520              
  Lines       20396    20396              
==========================================
+ Hits        16972    16974       +2     
+ Misses       3424     3422       -2     

Files with missing lines	Coverage Δ
...telemetry/sdk/metrics/state/async_metric_storage.h	93.19% <100.00%> (ø)
...ntelemetry/sdk/metrics/state/sync_metric_storage.h	84.94% <ø> (ø)
sdk/src/metrics/state/sync_metric_storage.cc	93.69% <100.00%> (ø)
sdk/src/metrics/state/temporal_metric_storage.cc	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[marcalff]

LGTM