Enable monitoring services on Gitaly on seperate server

doc/administration/gitaly/index.md

@@ -164,11 +164,21 @@ Git operations in GitLab will result in an API error.
    postgresql['enable'] = false
    redis['enable'] = false
    nginx['enable'] = false
-   prometheus['enable'] = false
    unicorn['enable'] = false
    sidekiq['enable'] = false
    gitlab_workhorse['enable'] = false
 
+   # If you don't want to run monitoring services uncomment the following (not recommended)
+   # alertmanager['enable'] = false
+   # gitlab_exporter['enable'] = false
+   # grafana['enable'] = false
+   # node_exporter['enable'] = false
+   # prometheus['enable'] = false
+
+   # Enable prometheus monitoring - comment out if you disable monitoring services above.
+   # This makes Prometheus listen on all interfaces. You must use firewalls to restrict access to this address/port.
+   prometheus['listen_address'] = '0.0.0.0:9090'
+
    # Prevent database connections during 'gitlab-ctl reconfigure'
    gitlab_rails['rake_cache_clear'] = false
    gitlab_rails['auto_migrate'] = false