Change StatusCode to 200 when no metrics are collected

[dszmigielski]

Fixes #3582.

Changes

If Prometheus Exporter have no metrics to collect it reports StatusCode 200 now instead of 204.

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 #3643 (c07397f) into main (872a52f) will increase coverage by 0.06%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3643      +/-   ##
==========================================
+ Coverage   87.22%   87.29%   +0.06%     
==========================================
  Files         282      282              
  Lines       10132    10132              
==========================================
+ Hits         8838     8845       +7     
+ Misses       1294     1287       -7     

Impacted Files	Coverage Δ
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	67.85% <100.00%> (ø)
....Prometheus.HttpListener/PrometheusHttpListener.cs	82.66% <100.00%> (ø)
src/OpenTelemetry/Logs/Pool/LogRecordSharedPool.cs	78.94% <0.00%> (-21.06%)	⬇️
...Propagators/OpenTelemetryPropagatorsEventSource.cs	87.50% <0.00%> (-12.50%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	73.52% <0.00%> (-8.83%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	97.65% <0.00%> (+0.78%)	⬆️
src/OpenTelemetry/Logs/OpenTelemetryLogger.cs	88.88% <0.00%> (+2.22%)	⬆️
...tpListener/Internal/PrometheusCollectionManager.cs	80.48% <0.00%> (+2.43%)	⬆️
...metryProtocol/Implementation/ActivityExtensions.cs	95.05% <0.00%> (+3.29%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️
... and 2 more

[reyang]

LGTM, would be even better to have a test case.