request for a new benchmark client

### Expected behavior

It would be great to have a benchmark client that does workloads with:
1) Open-loop workloads (fixed arrival rate rather than fixed number of clients, aka [coordinated omission](https://www.google.com/search?q=coordinated+omission))
2) Workloads with variance in arrival rates. Perhaps this builds on the support for open-loop, but the arrival rate should vary to see how RocksDB handles spikes

I don't know whether this should be added to [db_bench_tool](https://github.com/facebook/rocksdb/blob/main/tools/db_bench_tool.cc) as that already has a lot of code. But the implementor can figure that out.

### Actual behavior

[db_bench_tool](https://github.com/facebook/rocksdb/blob/main/tools/db_bench_tool.cc) does closed-loop workloads today via a fixed number of threads.