Nit fixes to prometheus asp.net core

[cijothomas]

part 2 of #3510

[codecov]

Codecov Report

Merging #3511 (04548bd) into main (084244c) will increase coverage by 0.10%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main    #3511      +/-   ##
==========================================
+ Coverage   86.67%   86.77%   +0.10%     
==========================================
  Files         275      275              
  Lines        9970     9963       -7     
==========================================
+ Hits         8641     8645       +4     
+ Misses       1329     1318      -11     

Impacted Files	Coverage Δ
.../PrometheusExporterApplicationBuilderExtensions.cs	100.00% <ø> (ø)
...rometheusExporterEndpointRouteBuilderExtensions.cs	100.00% <ø> (ø)
...metheus.AspNetCore/PrometheusExporterMiddleware.cs	67.85% <ø> (ø)
...Prometheus.AspNetCore/PrometheusExporterOptions.cs	50.00% <ø> (ø)
...Listener/Internal/PrometheusExporterEventSource.cs	27.77% <ø> (ø)
...heus.HttpListener/Internal/PrometheusSerializer.cs	78.21% <ø> (ø)
...s.HttpListener/Internal/PrometheusSerializerExt.cs	100.00% <ø> (ø)
...theusHttpListenerMeterProviderBuilderExtensions.cs	58.33% <66.66%> (ø)
...rometheusExporterMeterProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
...tpListener/Internal/PrometheusCollectionManager.cs	80.48% <100.00%> (ø)
... and 6 more