Add unicorn metrics to Prometheus

We now have Prometheus metrics endpoints in unicorn. These need to be added as jobs to the Prometheus config.

mentioned in merge request !1702 (merged)

mentioned in issue gitlab-com/infrastructure#2197 (closed)

marked the checklist item Unicorn as completed

changed title from Add unicorn and sidekiq metrics to Prometheus to Add unicorn metrics to Prometheus

changed the description

Going to split out sidekiq to a separate issue

closed