Summary:
Adding a list of blob db counters.

Also remove WaStats() which doesn't expose the stats and can be substitute by (BLOB_DB_BYTES_WRITTEN / BLOB_DB_BLOB_FILE_BYTES_WRITTEN).
Closes https://github.com/facebook/rocksdb/pull/3193

Differential Revision: D6394216

Pulled By: yiwu-arbug

fbshipit-source-id: 017508c8ff3fcd7ea7403c64d0f9834b24816803