Summary:
Adding sanity check test for mapping of `Histograms` and `HistogramsNameMap`
```
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from StatisticsTest
[ RUN ] StatisticsTest.SanityTickers
[ OK ] StatisticsTest.SanityTickers (0 ms)
[ RUN ] StatisticsTest.SanityHistograms
[ OK ] StatisticsTest.SanityHistograms (0 ms)
[----------] 2 tests from StatisticsTest (0 ms total)
[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (0 ms total)
[ PASSED ] 2 tests.
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4720
Differential Revision: D13217061
Pulled By: ajkr
fbshipit-source-id: 6427f4e684c36b2f3c3440808b74fee86a364683
1db4a096d4