Summary:
This PR supports querying `GetMapProperty()` with "rocksdb.dbstats" to get the DB-level stats in a map format. It only reports cumulative stats over the DB lifetime and, as such, does not update the baseline for interval stats. Like other map properties, the string keys are not (yet) exposed in the public API.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9057

Test Plan: new unit test

Reviewed By: zhichao-cao

Differential Revision: D31781495

Pulled By: ajkr

fbshipit-source-id: 6f77d3aee8b4b1a015061b8c260a123859ceaf9b