Throughput metric may be not well defined and can be improved

[adrianomg]

Real-time throughput metrics need to be refined and presented in more detail. It is currently implemented more like "ingestion rate" than actual throughput since it uses the source item counter to compute it. It does not use the sink item counter because operators can drop items before the sink. Maybe "ingestion rate" should be added as a different metric. It could also implement a monitor-throughput metric, to show it by operator, as the monitor-latency one. SPBench offers a way to customize the item counter, but that may not be enough and requires the user to add additional code to the benchmark.