Reported rate can be higher than actual when latency is high

[tgrabiec]

In the following output, reported rate is close to 4k rows/s but the actual rate is half that.

scylla-bench executes requests concurrently according to the configured concurrency and emits a partial result from each concurrent fiber if more than a second has elapsed. Those partial results are then summed up together and assumed to have been produced over a period of a second. When latency is high, the period is actually larger than a second, and actual rate lower.

time                operations/s   rows/s   errors  max              99.9th           99th             95th             90th             median           mean             
2.416392808s                4027     4027        0  1.023410175s     1.013972991s     1.007681535s     999.817215ms     996.147199ms     966.262783ms     747.089756ms     
4.448323544s                3485     3485        0  1.035468799s     1.031274495s     1.020788735s     1.014497279s     1.010827263s     993.525759ms     993.77632ms      
6.339966315s                3630     3630        0  1.040711679s     1.029177343s     1.019740159s     1.010302975s     1.002438655s     974.651391ms     964.700896ms     
8.189235084s                3734     3734        0  1.029177343s     1.024458751s     1.013972991s     1.004535807s     998.768639ms     923.271167ms     929.854113ms