Summary:
this PR adds two more per-level perf context counters to track
* number of keys returned in Get call, break down by levels
* total processing time at each level during Get call
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4617

Differential Revision: D12898024

Pulled By: miasantreble

fbshipit-source-id: 6b84ef1c8097c0d9e97bee1a774958f56ab4a6c4