This commit improves async op sanitizer speed by only delaying metrics
collection if there are pending ops. This
results in a speedup of around 30% for small CPU bound unit tests.
It performs this check and possible delay on every collection now,
fixing an issue with parent test leaks into steps.
The fix for #20188 was not entirely correct -- we were unlocking the
global buffer incorrectly. This PR introduces a lock state that ensures
we only unlock a lock we have taken out.
