[Exporter.Prometheus] Log when metric ignored

[martincostello]

Changes

Log a verbose message to the event log when a metric is ignored.

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
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.96%. Comparing base (7800d8c) to head (57c288b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...r/Internal/Shared/PrometheusExporterEventSource.cs	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7429      +/-   ##
==========================================
- Coverage   90.03%   89.96%   -0.07%     
==========================================
  Files         277      277              
  Lines       14628    14631       +3     
==========================================
- Hits        13170    13163       -7     
- Misses       1458     1468      +10     

Flag	Coverage Δ
unittests-Project-Experimental	89.88% <66.66%> (-0.07%)	⬇️
unittests-Project-Stable	89.89% <66.66%> (-0.06%)	⬇️
unittests-UnstableCoreLibraries-Experimental	48.85% <66.66%> (+0.06%)	⬆️

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

Files with missing lines	Coverage Δ
...ner/Internal/Shared/PrometheusCollectionManager.cs	90.26% <100.00%> (+0.05%)	⬆️
...ernal/Shared/Serialization/TextFormatSerializer.cs	100.00% <ø> (ø)
...r/Internal/Shared/PrometheusExporterEventSource.cs	48.14% <50.00%> (+0.32%)	⬆️

... and 4 files with indirect coverage changes