Improve test coverage for HttpClient instrumentation

[reyang]

@CodeBlanch suggested that we should have test coverage for the NuGet version of HttpClient open-telemetry/opentelemetry-dotnet#4912 (comment)

It seems 4.1.0, 4.3.0 and 4.3.4 are still being actively used https://www.nuget.org/stats/packages/System.Net.Http?groupby=Version.

[reyang]

@vishweshbankwar could you look into this?

[vishweshbankwar]

dotnet/runtime#86667 Based on discussions on this, I think we should not pursue this. A note in our Readme should be good enough that this scenario is not supported.

[reyang]

dotnet/runtime#86667 Based on discussions on this, I think we should not pursue this. A note in our Readme should be good enough that this scenario is not supported.

@CodeBlanch see if you agree - if yes, close the issue.

[CodeBlanch]

@ViktorHofer's comment:

...we can't mark the package as deprecated as it is part of the supported netstandard1.x dependency graph.

Seems to be a supported scenario?

[ViktorHofer]

The package contains some API that isn't available in earlier .NET Framework versions than 4.7.2 hence we can't deprecate it yet. That said, the majority of .NET Framework customers are already on 4.7.2 or newer. Other still in support TFMs don't require this package and we recommend to remove it from the dependency graph.

Note that the download numbers above are misleading as this package gets automatically downloaded if ANY of the packages in the dependency graph still targets netstandard1.x without providing a newer framework. Usually, that package shouldn't get unless for those earlier .NET Framework versions as the inbox assembly is preferred.

I agree with @vishweshbankwar to indicate that using the package isn't a supported scenario.