Update Prometheus exporter name and unit processing

[JamesNK]

Fixes #4742
Design discussion issue #

Changes

Updates the .NET Prometheus exporter to follow the metric and unit naming rules.

See https://github.com/open-telemetry/opentelemetry-specification/blob/b2f923fb1650dde1f061507908b834035506a796/specification/compatibility/prometheus_and_openmetrics.md#otlp-metric-points-to-prometheus

Merge requirement checklist

• CONTRIBUTING guidelines followed (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)

[JamesNK]

cc @noahfalk @lmolkova

[codecov]

Codecov Report

Merging #4753 (d51688e) into main (031ed48) will increase coverage by 0.27%.
The diff coverage is 99.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4753      +/-   ##
==========================================
+ Coverage   82.12%   82.40%   +0.27%     
==========================================
  Files         313      314       +1     
  Lines       12783    12910     +127     
==========================================
+ Hits        10498    10638     +140     
+ Misses       2285     2272      -13     

Files Changed	Coverage Δ
...ometheus.HttpListener/Internal/PrometheusMetric.cs	99.23% <99.23%> (ø)
...tpListener/Internal/PrometheusCollectionManager.cs	77.77% <100.00%> (+1.95%)	⬆️
...heus.HttpListener/Internal/PrometheusSerializer.cs	81.25% <100.00%> (+0.41%)	⬆️
...s.HttpListener/Internal/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

[JamesNK]

There are some failing tests. Are they broken by this PR or flaky tests?

[utpilla]

There are some failing tests. Are they broken by this PR or flaky tests?

Not related to your PR. That's a flaky test.

Tracking issue #4763

[CodeBlanch]

LGTM with a CHANGELOG update

[JamesNK]

I'd like to get this in for the next OTel release. Also, I'd like to be able to use these changes right now from the nightly feed.

What work remains before this can be merged?

[CodeBlanch]

@JamesNK I'm happy to merge but I do think this is a substantial change we should mention in the logs:

• https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md
• https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md

Something like this should work...

## Unreleased

* Added support for unit and name conversion following the [OpenTelemetry Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/065b25024549120800da7cda6ccd9717658ff0df/specification/compatibility/prometheus_and_openmetrics.md?plain=1#L235-L240)
  ([#4753](https://github.com/open-telemetry/opentelemetry-dotnet/pull/4753))

[JamesNK]

Done.