Summary:
Currently, application may pass a statistics object to db but later
wants to reduce stats tracking overhead by setting stats level to
kExceptHistogramOrTimers (the current lowest level). Tickers will still
be incremented, causing up to 1% CPU. We can add a new lowest stats
level `kExceptTickers` to disable ticker incrementing as well, thus
reducing CPU cycles spent on tickers.
Test Plan (devserver):
```
make check
make clean
DEBUG_LEVEL=0 make db_bench
./db_bench -perf_level=1 -stats_level=0 -statistics -benchmarks=fillseq,readrandom -duration=120
```
Measure CPU util (%) before and after change:
CPU util by rocksdb::RecordTick: 1.1 vs (<0.1)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7329
Reviewed By: pdillinger
Differential Revision: D23434014
Pulled By: riversand963
fbshipit-source-id: 72ff0f02a192ac476d4b0044b9f37fd4a22ff0d4