[sdk-metrics] Exemplar concurrency updates

[CodeBlanch]

Relates to #5364

Changes

• Make ExemplarReservoir Offer & Collect methods safe to be called concurrently (spec requirement). A follow-up PR will update the MetricPoint code to actually make concurrent calls.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[codecov]

Codecov Report

Attention: Patch coverage is 74.07407% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 85.41%. Comparing base (6250307) to head (9cd586c).
Report is 145 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5465      +/-   ##
==========================================
+ Coverage   83.38%   85.41%   +2.03%     
==========================================
  Files         297      289       -8     
  Lines       12531    12529       -2     
==========================================
+ Hits        10449    10702     +253     
+ Misses       2082     1827     -255     

Flag	Coverage Δ
unittests	?
unittests-Instrumentation-Experimental	24.19% <0.00%> (?)
unittests-Instrumentation-Stable	24.25% <0.00%> (?)
unittests-Solution-Experimental	85.42% <74.07%> (?)
unittests-Solution-Stable	85.31% <74.07%> (?)

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

Files	Coverage Δ
...try/Metrics/Exemplar/FixedSizeExemplarReservoir.cs	100.00% <100.00%> (ø)
...trics/Exemplar/SimpleFixedSizeExemplarReservoir.cs	75.00% <83.33%> (ø)
src/OpenTelemetry/Metrics/Exemplar/Exemplar.cs	87.09% <62.50%> (-12.91%)	⬇️

... and 61 files with indirect coverage changes