Export OpenMetrics format for prometheus exporters

[robertcoltheart]

Fixes #
First part to addressing #3972

Changes

Export OpenMetrics format for Prometheus. See here.

• Add Accept parser and set correct Content-Type
• Write metric timestamps in OpenMetric format

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)

[reyang]

LGTM with one suggestion https://github.com/open-telemetry/opentelemetry-dotnet/pull/5107/files#r1414630802.

[codecov]

Codecov Report

Merging #5107 (015ad3b) into main (a963ec8) will increase coverage by 0.06%.
Report is 1 commits behind head on main.
The diff coverage is 96.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5107      +/-   ##
==========================================
+ Coverage   83.06%   83.13%   +0.06%     
==========================================
  Files         296      297       +1     
  Lines       12320    12369      +49     
==========================================
+ Hits        10234    10283      +49     
  Misses       2086     2086              

Flag	Coverage Δ
unittests	83.13% <96.66%> (+0.06%)	⬆️

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

Files	Coverage Δ
...tpListener/Internal/PrometheusCollectionManager.cs	79.04% <100.00%> (+1.26%)	⬆️
...etheus.HttpListener/Internal/PrometheusExporter.cs	100.00% <100.00%> (ø)
...s.HttpListener/Internal/PrometheusHeadersParser.cs	100.00% <100.00%> (ø)
...s.HttpListener/Internal/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)
....Prometheus.HttpListener/PrometheusHttpListener.cs	81.70% <100.00%> (+1.70%)	⬆️
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	70.00% <92.30%> (+7.93%)	⬆️
...heus.HttpListener/Internal/PrometheusSerializer.cs	80.80% <90.00%> (+0.79%)	⬆️

... and 6 files with indirect coverage changes

[utpilla]

nit: It's simpler to not have the try/catch block here. This is a test so it's okay for it to fail with the original exception. Anyone troubleshooting the test failure would realize what the issue was anyway.

[robertcoltheart]

In this case, I'm guessing its because there might be a potential conflict with an already open port? I can remove in the next PR in any case.

[utpilla]

In this case, I'm guessing its because there might be a potential conflict with an already open port?

The tests within a project are not run in parallel so it shouldn't be the case unless the listener instances are not getting disposed correctly.

[robertcoltheart]

Thanks very much! Will crack on with the next PR.

[utpilla]

Thanks very much! Will crack on with the next PR.

@robertcoltheart Thanks for your contribution!