Add gauge for master event processing time

To diagnose cases where e.g., the master is backlogged, looking at just event_queue_messages will only tell about the size of the queue, but diagnosing whether this is due to higher message arrival rate or slower processing requires complicated interference with other metrics.

We should provide metrics to characterize the time it takes to process messages in the queue, optimally with statistics over some window. This would allow better identification of slow requests.

We should also consider ways to characterizing the arrival rate via some metric with statistics.