Adds docs for rake task

doc/administration/raketasks/maintenance.md

@@ -286,3 +286,14 @@ To re-import the metrics you can run:
 ```shell
 sudo gitlab-rake metrics:setup_common_metrics
 ```
+
+## Import additional development metrics
+
+A lot of different types of metrics are supported, like multi line metrics, heatmaps,
+anomaly, etc.
+
+To import these custom metrics, you can run:
+
+```shell
+sudo gitlab-rake 'metrics:setup_development_metrics[your_project_id]'
+```