Summary: similar to D52809 add option to exclude zero counters.

Test Plan:
[yiwu@dev4504.prn1 ~/rocksdb] ./iostats_context_test
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from IOStatsContextTest
[ RUN      ] IOStatsContextTest.ToString
[       OK ] IOStatsContextTest.ToString (0 ms)
[----------] 1 test from IOStatsContextTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (0 ms total)
[  PASSED  ] 1 test.

Reviewers: anthony, yhchiang, andrewkr, IslamAbdelRahman, kradhakrishnan, sdong

Reviewed By: sdong

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D54591