Summary:
Add more stats for better visibility into the usefulness of the secondary cache.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11246

Test Plan: Add a new unit test

Reviewed By: akankshamahajan15

Differential Revision: D43521364

Pulled By: anand1976

fbshipit-source-id: a92f04884e738a9bf40ad4047acaaaea343838a7