Add Sidekiq dead job metrics

What does this MR do?

This adds a counter, sidekiq_jobs_dead_total, that increments when a job is added to the dead jobs queue. It has the same basic labels (worker, queue, feature category, etc.) as the other Sidekiq metrics. To do this, we use the Sidekiq death handler configuration option, which is global: https://github.com/mperham/sidekiq/wiki/Error-Handling#death-notification

The easiest way to test this is to just make a job that won't work (wrong arguments) locally in a console:

MergeWorker.perform_async('foo')
#=> "e49157fce745b9dca2681822"

After a short time it will show up at http://localhost:3000/admin/sidekiq/morgue:

And we can also see it in our new prometheus counter:

For gitlab-com/gl-infra/scalability#275 (closed).