Track queue time/event age for async dispatch

[bluekeyes]

It would be nice to have a github.event.age metric or similar, that reports statistics about the age (amount of time spent in the queue) of webhook events. Should only have to record the current time when adding to the queue and then compare it to the current time at the start or end of processing.

[jmcampanini]

It would also be nice to have a metric for the number of times the capacity limit was hit, to differentiate the 5xx errors from the capacity errors

[bluekeyes]

Good point, something like a github.event.dropped counter that is incremented when the queue is full.