[Not for merging] netcoreapp3.1 for Prometheus middleware project

[alanwest]

No need for merging or making a decision at the moment. Opening this PR just to illustrate things, but I plan to close it and just open an issue.

This needs to be an additional discussion point as we consider how to handle #3448.

The Prometheus ASP.NET Core middleware project previously targeted netcoreapp3.1. Upon merging our net7.0 branch, it now just targets net6.0.

Do we only want to support net6.0+ with our Prometheus exporter?

Another option may be to "support" it in the way this PR proposes by suppressing the build warnings for the purposes of our build, though I believe this would still result in end users getting the warnings.

Maybe we'd take the stance that we don't really support the netcoreapp3.1 version even though we offer a version that targets it?

[codecov]

Codecov Report

Merging #3559 (2032c16) into main (72f4e07) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3559      +/-   ##
==========================================
+ Coverage   87.23%   87.27%   +0.04%     
==========================================
  Files         275      275              
  Lines        9959     9959              
==========================================
+ Hits         8688     8692       +4     
+ Misses       1271     1267       -4     

Impacted Files	Coverage Δ
src/OpenTelemetry.Api/Internal/Guard.cs	0.00% <ø> (ø)
...Listener/Internal/PrometheusExporterEventSource.cs	16.66% <0.00%> (-11.12%)	⬇️
....Prometheus.HttpListener/PrometheusHttpListener.cs	78.66% <0.00%> (-4.00%)	⬇️
...tpListener/Internal/PrometheusCollectionManager.cs	78.04% <0.00%> (-2.44%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	94.50% <0.00%> (-1.10%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	77.50% <0.00%> (+0.83%)	⬆️
src/OpenTelemetry/BatchExportProcessor.cs	84.11% <0.00%> (+1.86%)	⬆️
...tation/OpenTelemetryProtocolExporterEventSource.cs	85.00% <0.00%> (+10.00%)	⬆️
src/OpenTelemetry/Logs/Pool/LogRecordSharedPool.cs	100.00% <0.00%> (+21.05%)	⬆️

[alanwest]

Closing to continue discussion on #3560