Metrics not cleaned up when resources are deleted

[JoelSpeed]

If a resource is removed from a repository, the GTO managing the resource is deleted. This does not however remove the metrics associated with the GTO and as such, if the resource was not in sync, that metric will remain not in sync (potentially causing alerts) until such a time as the controllers are restarted.

We should work out how to remove a metric from prometheus when we delete a GTO.