[Flask] request_hook for metrics attributes

[TedaLIEz]

What problem do you want to solve?

We want to customize the attributes of metrics sent via FlaskInstrumentor#instrument_app, we did this with spans via request_hook and response_hook, but we didn't find a way to do that with metrics.

Describe the solution you'd like

There is a request_hook and response_hook in FlaskInstrumentor#instrument_app, however, the hook itself only works for traces, which is span specifically.

I'd like to see that the request_hook can also apply to metrics, for which we can provide more context via the hook function object.

Describe alternatives you've considered

No response

Additional Context

No response

Would you like to implement a fix?

None