[Metrics SDK] Reduce the scope of write lock

[utpilla]

• Write lock is only needed to get hold of the right tracker
• We don't need to hold the write lock while updating the trackers. Updates for Histograms are more involved than other instruments. There is no need to hold the write lock for the duration of the update.

Changes

• Drop the RwLockWriteGuard once we have the right tracker and then update the tracker

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 80.64516% with 6 lines in your changes missing coverage. Please review.

Project coverage is 77.9%. Comparing base (42b4f2f) to head (da63650).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/mod.rs	80.6%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2489     +/-   ##
=======================================
- Coverage   77.9%   77.9%   -0.1%     
=======================================
  Files        123     123             
  Lines      22925   22947     +22     
=======================================
+ Hits       17870   17886     +16     
- Misses      5055    5061      +6     

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[utpilla]

This won't work properly for Delta temporality.