Add GitLab Registry metrics to Omnibus dashboard

Problem to solve

We recently added monitoring for the integrated GitLab registry: omnibus-gitlab!2884 (merged)

But we didn't add the metrics to the GitLab dashboard. We should do this.

Intended users

Further details

Proposal

Add a dashboard to the Omnibus dashboards for the relevant GitLab Registry metrics.

The Omnibus dashboards are located here: https://gitlab.com/gitlab-org/grafana-dashboards/tree/master/omnibus

Permissions and Security

Documentation

What does success look like, and how can we measure that?

Links / references