Update metrics docs

Update the list of GitLab metrics to include the missing metric names.

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -48,6 +48,8 @@ The following metrics are available:
 | upload_file_does_not_exist        | Counter   | 10.7 in EE, 11.5 in CE  | Number of times an upload record could not find its file |
 | failed_login_captcha_total        | Gauge | 11.0 | Counter of failed CAPTCHA attempts during login |
 | successful_login_captcha_total    | Gauge | 11.0 | Counter of successful CAPTCHA attempts during login |
+| unicorn_active_connections        | Gauge | 11.0 | The number of active Unicorn connections (workers) |
+| unicorn_queued_connections        | Gauge | 11.0 | The number of queued Unicorn connections |
 
 ### Ruby metrics