Code review comment for ~rharding/charm-grafana:add-mysql-datasource

A couple remarks inline. As well as this, I think it'd be valuable to have a Mojo spec to deploy grafana, etc., along with gypsy-danger and mysql and exercise the new grafana functionality so that we can CI it, although I don't know if that should be a blocker. There are a couple of likely-looking jobs on Mojo CI Jenkins, although neither passes currently:

https://jenkins.canonical.com/is-mojo-ci/job/live-is-charm-test-grafana-xenial/
https://jenkins.canonical.com/is-mojo-ci/job/live-is-prometheus/