+ add documentation for registry-exporter/metrics (...

+ add documentation for registry-exporter/metrics ( https://gitlab.com/gitlab-org/omnibus-gitlab/issues/3653 )

doc/administration/monitoring/prometheus/index.md

@@ -288,6 +288,12 @@ The PgBouncer exporter allows you to measure various PgBouncer metrics.
 
 [➔ Read more about the PgBouncer exporter.](pgbouncer_exporter.md)
 
+### Registry exporter
+
+The Registry exporter allows you to measure various Registry metrics.
+
+[➔ Read more about the Registry exporter.](registry_exporter.md)
+
 ### GitLab exporter
 
 The GitLab exporter allows you to measure various GitLab metrics, pulled from Redis and the database.

doc/administration/monitoring/prometheus/registry_exporter.md

+# Registry exporter
+
+> [Introduced](https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/2884) in GitLab 11.9.
+
+The Registry exporter allows you to measure various Registry metrics.
+To enable it:
+
+1. [Enable Prometheus](index.md#configuring-prometheus).
+1. Edit `/etc/gitlab/gitlab.rb` and enable [debug mode](https://docs.docker.com/registry/#debug) for the Registry:
+
+   ```ruby
+   registry['debug_addr'] = "localhost:5001"  # localhost:5001/metrics
+   ```
+
+1. Save the file and [reconfigure GitLab](../../restart_gitlab.md#omnibus-gitlab-reconfigure)
+   for the changes to take effect.
+
+Prometheus will now automatically begin collecting performance data from
+the registry exporter exposed under `localhost:5001/metrics`.
+
+[← Back to the main Prometheus page](index.md)