[aspnetcore] Restore metrics instrumentation in netstandard builds

[alanwest]

#2360 needs further discussion. It removes metric functionality from a supported target, so it is a breaking change that warrants a major version bump. That said, I don't see any reason to have removed this functionality at all.

This change needs to be reverted. Also, it appears this was released as 1.10.0. A version 1.10.1 will need to be released to fix this breaking change.

Lastly, I noted that #2360 was merged without the approval of an approver or maintainer.

@Kielek @CodeBlanch

[codecov]

Codecov Report

Attention: Patch coverage is 5.00000% with 38 lines in your changes missing coverage. Please review.

Project coverage is 73.82%. Comparing base (71655ce) to head (968f0ae).
Report is 643 commits behind head on main.

Files with missing lines	Patch %	Lines
...AspNetCore/Implementation/HttpInMetricsListener.cs	0.00%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2403      +/-   ##
==========================================
- Coverage   73.91%   73.82%   -0.09%     
==========================================
  Files         267       23     -244     
  Lines        9615      661    -8954     
==========================================
- Hits         7107      488    -6619     
+ Misses       2508      173    -2335     

Flag	Coverage Δ
unittests-Instrumentation.AspNet	76.73% <ø> (?)
unittests-Instrumentation.AspNetCore	70.33% <5.00%> (?)

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

Files with missing lines	Coverage Δ
...reInstrumentationMeterProviderBuilderExtensions.cs	100.00% <100.00%> (ø)
...AspNetCore/Implementation/HttpInMetricsListener.cs	0.00% <0.00%> (ø)

... and 273 files with indirect coverage changes

[Kielek]

There was a decision to fully drop .NET6/.NET7 support (without making. the major upgrade here: https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/2138/files#diff-9fc8151d477e5c83d4231a1da766f08e0f087e880e42e19695175598e4069b4a).
I do not think that dropping code which is officially not supported is something bad. IMO, it is even better to do this now, instead of just after release. The message is clear for end-users. What is more, it is sync with https://github.com/open-telemetry/opentelemetry-dotnet/blob/145e7ade6f3d518280d139624951822117b60ebd/README.md#supported-net-versions

If we speaking about lack of technical approval, I assumed that @CodeBlanch is fine with changes, just after fixing the feedback. #2360 (comment). Maybe I was wrong, if so, I am sorry for this.

[alanwest]

I do not think that dropping code which is officially not supported is something bad. IMO, it is even better to do this now, instead of just after release.

Our stance on .NET Standard was discussed at length a couple years ago. While I think your position is a valid one, it is not the one we settled on. Until we decide to change that, and discuss how, we need to stick with our prior decisions.

[CodeBlanch]

LGTM