Review the conditional compilations in ASPNET Core and other instrumentations.

[cijothomas]

Due to the addition of more targets like net5.0 and net6.0 to the instrumention projects, the conditional checks inside the code seems broken now.

example:
https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Instrumentation.AspNetCore/Implementation/HttpInListener.cs#L313 check for
#if NETSTANDARD2_1, means that code path is not triggered for net5.0 or net6.0

Opening an issue to do a round of reviews to make sure the conditional compilation flags are still correct, after the addition of new project targets.

[TimothyMothra]

@cijothomas In your PR, what was the reason for using #if NETSTANDARD2_1_OR_GREATER || NETCOREAPP3_1_OR_GREATER instead of just #if NETSTANDARD2_1_OR_GREATER ?

I would have expected the single to work, I'm curious if you found an issue with that.

[cijothomas]

Because the project had NETCOREAPP3.1/NET5.0/NET6.0 targets (along with netstandard), and I don't think NETSTANDARD2_1 covers that. Happy to correct myself, if this is not the case.

[martinjt]

Closing as a lot has changed in the last 2 years, it can be reopened if needed.