Summary:
Implement trace sampling to allow user to specify the sampling frequency, i.e. save one per how many requests, so that a user does not need to log all if he/she is interested in only a sampled set.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4963

Differential Revision: D14011190

Pulled By: tang-jianfeng

fbshipit-source-id: 078b631d9319b67cb089dd2c30e21d0df8dc406a