In the Top class, there is one lock that guards the global histogram metrics and the per collection statistics. These metrics are updated by separate methods and at different times during a query

Since these two pieces of the Top class are independent, they could be guarded by two different locks instead of one lock. This will reduce cache line and atomic concurrency.

POC: https://github.com/10gen/mongo/commit/3c3d7ae8ec93db1ebbb3da3f97f57235c71f5f1e#diff-966f199e499eb2a33268c3e408a24e1477608de7e4d0109641f6fadb31f12d2a