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
Merge pull request #7 from raisesistemas/latency

Add latency metric