Add path to metric for request latencies

[L7R7]

It would be great to have a label at the http_request_duration_seconds metric. That would make it possible to measure latencies for the different paths independently without losing the ability to aggregate them easily.

What do you think? If you're interested (and if it's not super hard to implement) I could try to come up with a PR for that

[qnikst]

I like the idea. And I'm open to the PR's to wai-middleware-prometheus, I don't have strong opinion on how that package should work, so will happily accept solutions that solve problems for people. I find inability to add labels to the http_request_duration_seconds as a great problem.