Expose in-built _request-observer_

[dchambers]

Is your feature request related to a problem? Please describe.
I'm trying to implement Prometheus according to my company's standard requirements, but this requires me to also provide details of outbound client network requests using the same metric as is used for in-bound server network requests -- using a role label of client instead of the server value used for in-bound requests.

Describe the solution you'd like
In just 3 hours I've been able to configure promster to meet the fairly exacting standards docs at my company except this one small detail 🙌. If the observeRequest request-observer was exposed on app.locals in the same way as prom-client is, then I'd now be fully compliant 😄.

Any chance you'd consider exposing this observer too? Thanks in advance, Dominic.

Describe alternatives you've considered
The only alternative I can think of is to fork as I don't think it's possible for me to grab this reference any other way.

Additional context
None.

[tdeekens]

Thanks for reporting. Doesnt sound impossible. I will verify this the coming days to see what I can do.

[tdeekens]

I added a PR at #48. Can you verify there if this is what you would need? Lastly, I don't think I fully understand what this enables you to do. Can you maybe explain a bit more with a concrete example? Thanks.

[tdeekens]

Release in https://github.com/tdeekens/promster/releases/tag/%40promster%2Fexpress%402.1.0