Summary:
Add counters to track block cache index/filter hits and misses. We currently count aggregate hits and misses, which includes index/filter/data blocks.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4540

Differential Revision: D10459652

Pulled By: anand1976

fbshipit-source-id: 0c59eee7f12f5103dcb6686f0e7995babe63d425