Group coordinator under-reports thread idle time

When events arrive at the event processor with less than group.coordinator.threads ms between them, the thread idle time is rounded down to 0 because of integer division.

eg. when the group coordinator has 8 threads, and it receives >1k offset commit requests per second evenly split across shards, the thread idle time metric will be under-reported.

https://github.com/apache/kafka/blob/2353a7c5081b53402d5092afd255e2bdf7aa2d20/coordinator-common/src/main/java/org/apache/kafka/coordinator/common/runtime/MultiThreadedEventProcessor.java#L141