Summary:
Add a getTotalCount() method to the Histogram class so that callers
can read out the number of values that were put into the histogram

Test Plan: Added a new test.

Reviewed By: simpkins@fb.com

Subscribers: net-systems@, evanmao, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2078239

Tasks: 7097793

Signature: t1:2078239:1431739799:5de3a02df67ff535e06b4a1547690440cf594824