Expose service metrics via Prometheus

Using https://github.com/prometheus/client_golang we can add an HTTP listener to gitlab-workhorse and expose counters etc. to be scraped by Prometheus. For instance, the number of requests served broken down by HTTP method.

Before I spend more time on this I would like to know who/what should have access to this HTTP endpoint. Not the whole world.

I think securing / configuring access to the new HTTP end point is the 'hard part' of adding Prometheus support to gitlab-workhorse.

@pcarranza do we already have something in place that scrapes Prometheus endpoints behind the firewall on gitlab.com boxes? If not what would this look like?

I don't think it makes sense to add the workhorse support for this until we understand what we need on gitlab.com.