Summary:
The NUM_INDEX_AND_FILTER_BLOCKS_READ_PER_LEVEL, NUM_DATA_BLOCKS_READ_PER_LEVEL, and NUM_SST_READ_PER_LEVEL stats were being recorded only when the last file in a level happened to have hits. They are supposed to be updated for every level. Also, there was some overcounting of GetContextStats. This PR fixes both the problems.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9583

Test Plan: Update the unit test in db_basic_test

Reviewed By: akankshamahajan15

Differential Revision: D34308044

Pulled By: anand1976

fbshipit-source-id: b3b36020fda26ba91bc6e0e47d52d58f4d7f656e