Display p95 or p99 of background job latency

[aaronjensen]

At the moment we can see average latency per job for background jobs. In order to know whether or not we need to add more background job processors we want to know what the p95 or p99 latency is for jobs. Average could stay very low, but if we're striving for a near 0 latency then p95 would be more telling.

Thanks!

[maia]

I'd additionally like to see p95/p99 for execution time, or the percentage of jobs running longer than n seconds.