fix inclusive-exclusiveness of histogram ToString

[ajkr]

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:

rocksdb/monitoring/histogram.cc

Line 53 in 867fe92

valueIndexMap_.lower_bound(value);

. Fixed histogram's ToString() to reflect this.

Test Plan:

• unit test to verify in which bucket boundary values are added
• verified db_bench histogram output:

[       0,       1 ]    81936   8.194%   8.194% ##
(       1,       2 ]   636865  63.687%  71.880% #############
(       2,       3 ]   237846  23.785%  95.665% #####
(       3,       4 ]    34207   3.421%  99.085% #
(       4,       6 ]     5712   0.571%  99.657% 
(       6,      10 ]      914   0.091%  99.748% 
(      10,      15 ]       50   0.005%  99.753% 
...

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@ajkr updated the pull request - view changes - changes since last import

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