Improve the experience of using Gitaly and GDK

What does this Merge Request do?

While trying to get Prometheus working with Gitaly and GDK, so I could try out: https://docs.gitlab.com/ee/administration/gitaly/#observe-type-of-gitaly-connections, I ran into a problems understanding the right way to configure Gitaly for Prometheus in the default GDK configuration.

The prometheus.yml we offered doesn't work out of the box for Gitaly, and doesn't offer the "casual Prometheus user" any help with getting it to work. It also didn't have Praefect configuration, when Praefect comes "for free" in every GDK install.

This MR:

• Solves the problem I had, which is making it easy for a first timer to get Prometheus and Gitaly/Praefect talking.
• Makes gdk.test hostname more of a thing.
• Does a light tidy up of the GDK Gitaly "how to".

The MR doesn't contribute to solving any part of: #827, where the configuration customizations to Gitaly and Praefect are lost after a gdk reconfigure.

CC @jramsay @ashmckenzie @zj-gitlab @mikelewis @deuley

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.