* FIX: ensure threads do not leak when #start is called
Periodic instrumentation now follows a general pattern where they
inherit from PeriodicStats.
This implementation ensures proper management of backing thread
1. All classes inheriting now have a .started? method to determine
if instrument is started
2. All classes implement a clean .stop method for stopping active
periodic instrumentation
3. Consumers of the class get to write less code repeat code due
to worker_loop
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