Summary:
Right now when a flush is triggered, the memory consumption is logged but data size is not.
It's useful to log both when we debug unexpected small flushed file size.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4979

Differential Revision: D14071979

Pulled By: siying

fbshipit-source-id: 0cd60449c5205eb00e0fbc299084418f609904ed