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.
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