[otlp-metrics] Support exemplars for all metric types

[CodeBlanch]

Changes

• Support exemplars for all metric types in OtlpMetricExporter

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

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 84.61%. Comparing base (6250307) to head (19477a5).
Report is 103 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5397      +/-   ##
==========================================
+ Coverage   83.38%   84.61%   +1.22%     
==========================================
  Files         297      284      -13     
  Lines       12531    12100     -431     
==========================================
- Hits        10449    10238     -211     
+ Misses       2082     1862     -220     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	84.52% <85.71%> (?)
unittests-Solution-Stable	84.45% <85.71%> (?)

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

Files	Coverage Δ
...tryProtocol/Implementation/MetricItemExtensions.cs	91.58% <85.71%> (+9.57%)	⬆️

... and 53 files with indirect coverage changes