Merge branch 'docs-unicorn-metrics' into 'master'

Use title: "Unicorn Metrics available" as in EE

See merge request gitlab-org/gitlab-ce!18620

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -22,7 +22,7 @@ collect metrics from this endpoint. We recommend setting up another Prometheus
 server, because the embedded server configuration is overwritten once every
 [reconfigure of GitLab][reconfigure]. In the future this will not be required.
-## Metrics available
+## Unicorn Metrics available
 The following metrics are available: