Stats for allocation garbage collection

Nomad currently doesn't clean up orphaned garbage collection and there is no way for operators to know if it's cleaning up allocation directories unless disk space starts running out.

We need stats which indicate client side garbage collection success and failure rates.

I can submit a PR in the next few days.

No files, tests, or entry points are named. Start by locating Nomad's client-side garbage collection and its existing statistics or metrics, then determine how to expose success and failure rates so operators can observe cleanup activity.

Written by the indexing model from the issue text.