Summary:
The multireadrandom benchmark, when run for a specific number of reads (--reads argument), should base the duration on the actual number of keys read rather than number of batches.

Tests:
Run db_bench multireadrandom benchmark

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7817

Reviewed By: zhichao-cao

Differential Revision: D25717230

Pulled By: anand1976

fbshipit-source-id: 13f4d8162268cf9a34918655e60302d0aba3864b