Document how to add custom metrics to the self monitoring project

doc/administration/monitoring/gitlab_self_monitoring_project/index.md

@@ -48,3 +48,10 @@ to the Prometheus config in order for GitLab to receive notifications of any ale
 
 Once the webhook is setup, you can
 [take action on incoming alerts](../../../user/project/integrations/prometheus.md#taking-action-on-incidents-ultimate).
+
+## Adding custom metrics to the self monitoring project
+
+You can add custom metrics in the self monitoring project by:
+
+1. [Duplicating](../../../user/project/integrations/prometheus.md#duplicating-a-gitlab-defined-dashboard) the default dashboard.
+1. [Editing](../../../user/project/integrations/prometheus.md#view-and-edit-the-source-file-of-a-custom-dashboard) the newly created dashboard file and configuring it with [dashboard YAML properties](../../../user/project/integrations/prometheus.md#dashboard-yaml-properties).