Praefect docs: add instructions for setting up prometheus/grafana for gitaly and praefect nodes
In https://docs.gitlab.com/ee/administration/gitaly/praefect.html we have instructions for a basic Praefect deployment which we use for test drives when we do a demo. In recent demos it has been apparent that getting metrics for Praefect is not straightforward.
I suggest we expand the setup instructions in that document so that the end result is that we can use prometheus or grafana to run run queries against all gitaly and praefect nodes. This is what we expect anyway, in a production environment.