Implemented a cleanup worker for deleting metrics

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.
If it's only ever called for the initial environment, because it's a state-server-level method, then I can live with that -- but I still think that the inconsistency of skipping args isn't worth it. As humans we're notoriously bad at predicting the future in general, and how usage patterns will evolve in particular.