Add metric cleanup worker

There are 3 main changes in this pr:

1) Added a cleanup metrics and delete functions to state to allow metrics to be deleted
2) Added an API endpoint for calling the CleanupMetrics function
3) Added a worker that will periodically call the cleanupmetric function

The worker never gets called - except in tests - that will come in a follow up pr.
The CleanupMetrics function currently just returns an error. In a follow up it will return a list of the uuids that were deleted.

This pr follows on from: https://github.com/juju/juju/pull/562