Summary:
I spent too much time thinking about histograms lately and realized boundary values fall into the lower bucket, not the upper bucket. It's because we're using `std::map::lower_bound` here: 867fe92e5e/monitoring/histogram.cc (L53). Fixed histogram's `ToString()` to reflect this.
Closes https://github.com/facebook/rocksdb/pull/2817
Differential Revision: D5751159
Pulled By: ajkr
fbshipit-source-id: 67432bb45849eec9b5bcc0d095551dbc0ee81766
3b9a000c15