[prometheus] Fix OpenMetrics format suffixes

[robertcoltheart]

Fixes #5502

Changes

According to the OpenMetrics specs, unit must be a suffix of the metric name, and not just infix in the name. This PR also complies with the requirement that counters MUST end with _total, although this clashes with the OTEL spec that says it should be optional. Additionally, UNIT, HELP and TYPE metadata should not include the _total suffix, as seen in examples in the OpenMetrics specs.

Tested with the latest version of Prometheus.

See:

• https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#unit
• https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#counter-1

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 20.12%. Comparing base (6250307) to head (2fc263d).
Report is 243 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5646       +/-   ##
===========================================
- Coverage   83.38%   20.12%   -63.27%     
===========================================
  Files         297      185      -112     
  Lines       12531     7917     -4614     
===========================================
- Hits        10449     1593     -8856     
- Misses       2082     6324     +4242     

Flag	Coverage Δ
unittests	?
unittests-UnstableCoreLibraries-Experimental	20.12% <100.00%> (?)

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

Files	Coverage Δ
...ometheus.HttpListener/Internal/PrometheusMetric.cs	74.12% <100.00%> (+2.37%)	⬆️
...heus.HttpListener/Internal/PrometheusSerializer.cs	85.71% <100.00%> (+0.84%)	⬆️
...s.HttpListener/Internal/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)

... and 242 files with indirect coverage changes

[cijothomas]

@robertcoltheart Thanks a lot for taking good care of Prometheus exporters!

[CodeBlanch]

@robertcoltheart I'm happy to merge this because there are approvals but there is also some open feedback from @dashpole. Are you going to respond or make more changes or should I merge?

[robertcoltheart]

Let me make the suggested changes, and then we can review again