[prometheus] Fix issue with corrupted buffers when reading both OpenMetrics and plain text formats

[robertcoltheart]

Fixes #5515

Changes

Separate prometheus buffers into plain text and openmetrics. This is the first part of splitting #5517. It also optimizes the generation of the buffers, and only generates the requested buffer, rather than generating both simultaneously.

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

[codecov]

Codecov Report

Attention: Patch coverage is 76.59574% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 85.71%. Comparing base (6250307) to head (2e62fde).
Report is 229 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5623      +/-   ##
==========================================
+ Coverage   83.38%   85.71%   +2.33%     
==========================================
  Files         297      254      -43     
  Lines       12531    11036    -1495     
==========================================
- Hits        10449     9460     -989     
+ Misses       2082     1576     -506     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	85.70% <76.59%> (?)
unittests-Solution-Stable	85.69% <76.59%> (?)
unittests-Unstable-Core	19.95% <76.59%> (?)

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

Files	Coverage Δ
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	70.73% <100.00%> (+0.73%)	⬆️
....Prometheus.HttpListener/PrometheusHttpListener.cs	83.13% <100.00%> (+1.42%)	⬆️
...tpListener/Internal/PrometheusCollectionManager.cs	77.93% <73.17%> (+0.15%)	⬆️

... and 113 files with indirect coverage changes

[reyang]

Thanks @robertcoltheart!

[robertcoltheart]

Done, please review

[vishweshbankwar]

LGTM - Thanks @robertcoltheart!

[bbrandt]

(orig)
This change results in a regression causing no output to be displayed when IncreaseBufferSize(...) is needed. The issue seems to be that IncreaseBufferSize(...) is taking a reference to the buffer local variable, but not to this.openMetricsBuffer or this.plainTextBuffer, so these members do not get the updated content after a resize occurs. Then on 294 when we try to access this.openMetricsBuffer or this.plainTextBuffer it is the original buffer instead of the resized buffer.

Looks like this is fixed by #5676, which has not yet been merged.

@Kielek @vishweshbankwar @CodeBlanch What is needed to get #5676 merged?