[gorouter] Support Prometheus metrics

[maxmoehl]

Proposed Change

Extend the prometheus endpoint in gorouter to export all metrics which are currently exported only via envelopes v1. There was previous work in #246 (maybe @Benjamintf1 or @stefanlay have something to add here) and I've started a POC at https://github.com/sap-contributions/gorouter/compare/maxmoehl/prom. As you can see the interface provided by the prometheus library is a bit lacking and we would probably need to extend it.

One thing to note is that, if you use the graphite nozzle, tags will not work because those are not supported by the graphite nozzle. The main question I'd like to clarify upfront is whether we keep using that library or simply consume prometheus directly?

Acceptance criteria

As a operator
I can switch envelopes v1 and prometheus on/off independently
So that I can choose the right metric collection approach for my system.

Related links

No response

[maxmoehl]

Regarding the go-metric-registry: Should we:

1. extend it to cover the use-cases we have for prometheus, or
2. drop it and consume prometheus directly?

I'm in favour of using prometheus directly primarily because I don't see what the library is abstracting away. There is only this one implementation and the support from the prometheus community seems to be very good.