Better server route metrics labels and trace operation names

[jtlisi]

I recently read https://www.robustperception.io/prometheus-middleware-for-gorilla-mux and noticed the usage of the actual url path was used with forward slashes in the label. I may be alone in this but I think having labels with the actual URL and using the actual url for operation name in a trace is more ergonomic than the underscore.

For example:

HTTP GET - api_prom_api_v1_query_range
vs
HTTP GET - /api/prom/api/v1/query_range

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[bboreham]

Note that it isn't exactly the url, since it can have placeholders.
I don't know why / is replaced by _. It feels unusual to have / in a Prometheus label, but there is no rule against it (or in OpenTracing, as far as I can see).

[pracucci]

We should pay attention because we may break dashboards and alerts of our users. Could be a candidate for Cortex 2.0.