Summary:
- Clarified some comments on compatibility for adding new ticker stats
- Added read I/O stats for `VerifyChecksum()` and `VerifyFileChecksums()` APIs
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8741
Test Plan: new unit test
Reviewed By: zhichao-cao
Differential Revision: D30708578
Pulled By: ajkr
fbshipit-source-id: d06b961f7e199ae92c266b683e39870aa8f63449