[Exporter.Prometheus] Refactor Prometheus serializer

[martincostello]

Contributes to #5725, #7155 and #7246.

Changes

• Implement the logic for the new serializer design and wire into PrometheusCollectionManager.
• Update the tests to call the new serializer instead of the old one.
• Delete the old PrometheusSerializer class.
• Improve test coverage.
• Remove dead code.

WIP as I need to run the benchmarks and check for any clean-up.

The idea is that by splitting things out of the giant monolith we have today, text-format specific behaviours are easier to implement in future PRs.

When ready for review this will probably be easiest to review commit-by-commit (add new code, move tests, delete old code).

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 90.26%. Comparing base (184b1cf) to head (a68e49b).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7412      +/-   ##
==========================================
+ Coverage   89.74%   90.26%   +0.52%     
==========================================
  Files         276      279       +3     
  Lines       14623    15360     +737     
==========================================
+ Hits        13123    13865     +742     
+ Misses       1500     1495       -5     

Flag	Coverage Δ
unittests-Project-Experimental	89.91% <100.00%> (+0.18%)	⬆️
unittests-Project-Stable	89.86% <100.00%> (+0.16%)	⬆️
unittests-UnstableCoreLibraries-Experimental	48.78% <100.00%> (+0.30%)	⬆️

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	90.20% <100.00%> (ø)
...rnal/Shared/Serialization/OpenMetricsSerializer.cs	100.00% <100.00%> (ø)
...l/Shared/Serialization/PrometheusTextSerializer.cs	100.00% <100.00%> (ø)
...ernal/Shared/Serialization/TextFormatSerializer.cs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes