MLPerf-Inference Server: change metrics from scheduled to completed samples per second for all benchmarks

[nvzhihanj]

As presented in https://docs.google.com/presentation/d/1Y_AKEJ6h1g5k3ntrL7nTazWw3xVDzJ_tjOGkLQ6VDMI/edit?usp=sharing the completed sample per second is a better representation of the throughput than scheduled QPS. @pgmpablo157321 to help implement after the conclusion of v4.0