Summary:
There's an existing benchmark, "getmergeoperands", but it is unconventional in that it has multiple phases and hardcoded setup parameters.
This PR adds a different one, "readrandomoperands", that follows the pattern of other benchmarks of having a single phase and taking its configuration from existing flags.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9785
Test Plan:
```
$ ./db_bench -benchmarks=mergerandom -merge_operator=StringAppendOperator -write_buffer_size=1048576 -max_bytes_for_level_base=4194304 -target_file_size_base=1048576 -compression_type=none -disable_auto_compactions=true
$ ./db_bench -use_existing_db=true -benchmarks=readrandomoperands -merge_operator=StringAppendOperator -disable_auto_compactions=true -duration=10
...
readrandomoperands : 542.082 micros/op 1844 ops/sec; 0.2 MB/s (11980 of 18999 found)
```
Reviewed By: jay-zhuang
Differential Revision: D35290412
Pulled By: ajkr
fbshipit-source-id: fb367ca614b128cef844a75f0e5d9dd7c3328d85
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