Summary:
The `rocksdb.db.get.micros` histogram is never updated if the DB is open in ReadOnly mode, as well as the `get_cpu_nanos` perf counter. An earlier PR (https://github.com/facebook/rocksdb/issues/4260) for some reason has only added the TODO line, not the accounting itself, so this one is intended to fix it, adding two lines to match [DBImplSecondary](https://github.com/facebook/rocksdb/blob/4dafa5b220c439b7f05eb598a2ab32b3152bedb5/db/db_impl/db_impl_secondary.cc#L366-L367).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11521

Reviewed By: cbi42

Differential Revision: D46577330

Pulled By: jowlyzhang

fbshipit-source-id: be147923e763af32bbc18fd6bdf3aff8ebf08aee