We set the atomic _isEnabled to false here when shutting down the fast count thread.

The fast count thread waits on a condition variable here that checks the value of _flushRequested and _isEnabled.

Shutdown sets _isEnabled to false without holding the metadata mutex. If the thread is started up and shutdown quickly, we can have a lost wakeup where the condition variable does not catch that isEnabled is set to false before entering the wait loop, and unless it spuriously wakes up and checks this value which is not guaranteed to happen, the thread will continue waiting indefinitely and shutdown will wait for the thread to be joined.