Add clusterrole appuio:metrics-viewer

[glrf]

This clusterrole can be used to delegate access to the user-workload monitoring metrics. By binding this clusterrole in a namespace it gives the subjects access to all metrics in that namespace

Checklist

• PR contains a single logical change (to build a better changelog).
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog.