Use max thread pool size instead of current size for 'utilization' metric

I believe the purpose of this metric is to determine when the Jetty thread pool is short on idle threads for servicing incoming requests. We currently alert on the previous version of this metric (percent-idle), however this proved to be useless as when the thread pool is short on threads, it will create more up to the maximum which causes this metric to fall back below the alert threshold we had set.

If the current implementation of the utilization metric is important, possibly create a new metric to track idle/max for determining when the service will actually stop being able to service requests due to lack of idle threads.