Sidekiq middleware prometheus metrics needs to be documented

The following discussion from !17682 (merged) should be addressed:

• @tkuah started a discussion:

  @qzhaogitlab I was going to ask if we need to update docs but I noticed all the metrics from this middleware seems to be missing from https://gitlab.com/gitlab-org/gitlab/blob/master/doc%2Fadministration%2Fmonitoring%2Fprometheus%2Fgitlab_metrics.md. So will spin this off into its own issue