[sdk-metrics] Support setting exemplar filter from spec envvar key

[CodeBlanch]

Changes

• Adds support for configuring exemplar filter via OTEL_METRICS_EXEMPLAR_FILTER.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 84.71%. Comparing base (6250307) to head (ec4849e).
Report is 112 commits behind head on main.

❗ Current head ec4849e differs from pull request most recent head aa7d236. Consider uploading reports for the commit aa7d236 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5412      +/-   ##
==========================================
+ Coverage   83.38%   84.71%   +1.33%     
==========================================
  Files         297      281      -16     
  Lines       12531    12097     -434     
==========================================
- Hits        10449    10248     -201     
+ Misses       2082     1849     -233     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	84.70% <100.00%> (?)
unittests-Solution-Stable	84.68% <100.00%> (?)

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

Files	Coverage Δ
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	95.08% <100.00%> (+7.24%)	⬆️
src/OpenTelemetry/Metrics/MetricReaderExt.cs	91.42% <100.00%> (+1.42%)	⬆️

... and 55 files with indirect coverage changes

[cijothomas]

left a nit about changelog being more self-contained, but that is not a 🚫