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
Proper exception handling

Catching a generic exception may be misleading. Besides being unable
to stablish a connection to Redis, other reasons might cause
the monitor to crash and it's important that they get properly reported.

One strong indicator that the current exception handling can be improved is
the unused variable `e`.
Custom agent guid

This is necessary to use the custom Latency metric while upstream
doesn't accept the PR.

https://github.com/eksoverzero/newrelic_sidekiq_agent/pull/7
Merge branch 'custom-agent-guid'
Merge branch 'proper-exception-handling'
Merge branch 'latency'