Prometheus gauge for in-flight requests

[yabberyabber]

I would really like to be able to track the number of requests that are currently in flight for my application.

To do this, I could of course create my own middleware that wraps each request, but with this approach it would be difficult for my custom metric to match the naming convention and label-set as those metrics provided by the prometheus package in this repo.

Would the maintainers here be open to a PR from me where I add an requests_in_flight gauge to track the number of currently running packages?