Add latency metric

[Latency is the time elapsed since the oldest job on the queue has been
enqueued](1). This is a metric that can become handy to detect stale
Sidekiq workers.

Sidekiq ~> 2.12 is required to use this metric, as stated in the [Changelog](2).

[1]: https://github.com/mperham/sidekiq/wiki/Monitoring#monitoring-queue-latency
[2]: https://github.com/mperham/sidekiq/blob/master/Changes.md#2121