[prometheus] Support meter-level tags

[robertcoltheart]

Fixes #
#5836

Changes

Adds meter-level tags to Prometheus exporter. Any tags added to meter are automatically added to scrape output.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 86.30%. Comparing base (6250307) to head (45cb7fc).
Report is 322 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5837      +/-   ##
==========================================
+ Coverage   83.38%   86.30%   +2.91%     
==========================================
  Files         297      257      -40     
  Lines       12531    11172    -1359     
==========================================
- Hits        10449     9642     -807     
+ Misses       2082     1530     -552     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.25% <100.00%> (?)
unittests-Project-Stable	85.92% <100.00%> (?)
unittests-UnstableCoreLibraries-Experimental	85.75% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...heus.HttpListener/Internal/PrometheusSerializer.cs	87.15% <100.00%> (+2.28%)	⬆️

... and 224 files with indirect coverage changes

[ProbablePrime]

Great job!

[ysolomchenko]

LGTM

[rajkumar-rangaraj]

I see that the current tests have been updated to validate MeterTags. Is there a test that validates null MeterTags?"

[robertcoltheart]

I've added some additional tests as requested. I think a follow-up PR is required to refactor these tests a bit, since we have quite a few different combinations of conditions.