Implement support for metrics in HttpClient for .NET framework

[pyohannes]

Feature Request

Is your feature request related to a problem?

Currently, HttpClient instrumentation supports metrics for .NET (Core), but not for .NET framework. This is mentioned in a warning in the documentation.

This is unexpected and seen as a bug by users I've interacted with. Users of e. g. the auto-instrumentation packages or other instrumentation that packages that use HttpClient under the hood will likely not find the warning.

Describe the solution you'd like:

Adding support for metrics also for .NET framework would be a desired solution.

Describe alternatives you've considered.

Users who currently face this issue have to either provide their own instrumentation library, add manual instrumentation, or create metrics from fully sampled traces.

Additional Context

Related Slack discussion
Warning in the documentation

[matt-hensley]

I've started on a proposed PR for this and will have a draft ready next week.

[alejandrocarrasquilla]

@matt-hensley Any updates on this change?
We are interested in this functionality. We are specially interested in the implementation that does not rely on having traces enabled.
Thanks

[Jazaret]

Is there any update on this? Our team is depending on HttpClient metrics for .NET Framework.

[matt-hensley]

After feedback from maintainers on #4768 I am taking a different approach for the metrics implementation.

This should allow for generation of traces and metrics independently, as opposed to the previous PR that required tracing to be enabled, and for a particular trace to be sampled and recorded.

Currently iterating on this new approach and working through the tests as the current instrumentation is only trace focused.