Instrument http_request_duration_seconds metric with exemplars

[yeya24]

From the discussion #3938, we think it is a good start to instrument https://github.com/thanos-io/thanos/blob/main/pkg/extprom/http/instrument_server.go#L44 with Exemplars.

Exemplar example:

1. Add exemplar support to middleware weaveworks/common#202

Help is welcomed!

[metalmatze]

I am going to assign this to myself as I got it working from a Thanos standpoint for the most part. 🎉

Right now I'm unable to actually make it work in Grafana, but that should be all for the most part.

Probably, worth noting that I had to use the histogramVec directly as client_golang ObserveWithExemplar doesn't work with currying right now. I'll open an issue.