[Exporter.Prometheus] PrometheusSerializerTests - typo fixes

[Kielek]

Fixes N/A

Changes

Part of changes from: #3694

[codecov]

Codecov Report

Merging #3715 (4cd2e37) into main (92fb3b3) will increase coverage by 0.17%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3715      +/-   ##
==========================================
+ Coverage   87.20%   87.38%   +0.17%     
==========================================
  Files         274      274              
  Lines       10009    10009              
==========================================
+ Hits         8728     8746      +18     
+ Misses       1281     1263      -18     

Impacted Files	Coverage Δ
...tation/OpenTelemetryProtocolExporterEventSource.cs	75.00% <0.00%> (-10.00%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	96.87% <0.00%> (-0.79%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	82.35% <0.00%> (+2.94%)	⬆️
...metryProtocol/Implementation/ActivityExtensions.cs	95.05% <0.00%> (+3.29%)	⬆️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	77.27% <0.00%> (+40.90%)	⬆️
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	78.57% <0.00%> (+42.85%)	⬆️