Summary: Measuring mutex duration will measure time inside DB mutex, which breaks our best practice. Add a stat level in Statistics class. By default, disable to measure the mutex operations.
Test Plan: Add a unit test to make sure it is off by default.
Reviewers: rven, anthony, IslamAbdelRahman, kradhakrishnan, andrewkr, yhchiang
Reviewed By: yhchiang
Subscribers: MarkCallaghan, leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D53367
d20915d52a