[sdk-metrics] Expose Exemplars stable

[CodeBlanch]

Relates to #2527

Changes

• Exposes the Exemplar APIs previously covered by OTEL1002 as part of the stable API

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• 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 85.79%. Comparing base (6250307) to head (385278e).
Report is 220 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5607      +/-   ##
==========================================
+ Coverage   83.38%   85.79%   +2.40%     
==========================================
  Files         297      254      -43     
  Lines       12531    10982    -1549     
==========================================
- Hits        10449     9422    -1027     
+ Misses       2082     1560     -522     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.77% <ø> (?)
unittests-Solution-Stable	85.70% <ø> (?)
unittests-Unstable-Core	19.85% <ø> (?)

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

Files	Coverage Δ
.../Metrics/Builder/MeterProviderBuilderExtensions.cs	98.42% <ø> (-1.58%)	⬇️
src/OpenTelemetry/Metrics/Exemplar/Exemplar.cs	87.09% <ø> (-12.91%)	⬇️
...try/Metrics/Exemplar/ReadOnlyExemplarCollection.cs	78.57% <ø> (ø)
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	95.08% <ø> (+7.24%)	⬆️
src/OpenTelemetry/Metrics/MetricPoint.cs	94.24% <ø> (+25.76%)	⬆️
src/OpenTelemetry/ReadOnlyFilteredTagCollection.cs	100.00% <ø> (ø)

... and 108 files with indirect coverage changes