Support Prometheus UNIT metadata

[reyang]

Changes

• Added support for UNIT metadata.
• Renamed some of the internal methods to better align with OpenMetrics terms (e.g. "metadata").
• Adjusted the ordering of the metadata to follow the OpenMetrics best practices.

Reference: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#overall-structure

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #3651 (3d1fb73) into main (de98eb7) will increase coverage by 0.02%.
The diff coverage is 94.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3651      +/-   ##
==========================================
+ Coverage   87.58%   87.60%   +0.02%     
==========================================
  Files         283      283              
  Lines       10218    10234      +16     
==========================================
+ Hits         8949     8966      +17     
+ Misses       1269     1268       -1     

Impacted Files	Coverage Δ
...heus.HttpListener/Internal/PrometheusSerializer.cs	80.34% <93.75%> (+2.12%)	⬆️
...s.HttpListener/Internal/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)
...tpListener/Internal/PrometheusCollectionManager.cs	78.04% <0.00%> (-2.44%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+8.82%)	⬆️

[cijothomas]

nice description :)

[CodeBlanch]

Needs some clever Greek mythology thing. Defer to @alanwest to come up with it 🤣

[alanwest]

I mean, would "Hello, Prometheus!" not have been clever enough? And also what the heck do I know about Greek mythology?

[reyang]

Do we consider the Prometheus Exporters to be renamed to OpenMetrics Exporters? 😆

[CodeBlanch]

LGTM