Summary:
Add last level and non-last level read statistics:
```
LAST_LEVEL_READ_BYTES,
LAST_LEVEL_READ_COUNT,
NON_LAST_LEVEL_READ_BYTES,
NON_LAST_LEVEL_READ_COUNT,
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9519

Test Plan: added unittest

Reviewed By: siying

Differential Revision: D34062539

Pulled By: jay-zhuang

fbshipit-source-id: 908644c3050878b4234febdc72e3e19d89af38cd