Add static 'site' label to all metrics

[ccremer]

Summary

• Adds a new static label site to all metrics. Defaults to hostname in the URL if not given for backwards compatibility.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.