Added better counters to surosinkstat for KafkaSinkV2

These new counters are important for testing and monitoring a production deployment.
I did see increased throughput up to 25 threads on my single-core test VM.  With 25 threads I got a throughput of 20,000 100kb messages per second.  In this config file, I set corePoolSize to 50 to avoid creating a bottleneck on bigger machines.  For the thread pool size I thought it was better to err on the side of making it too large because "oversizing" would have less performance impact than "undersizing".  This is an important parameter and I included the comment to draw attention.