Update documents for external prometheus server setting

doc/administration/high_availability/monitoring_node.md

@@ -71,6 +71,19 @@ Omnibus:
 
 1. Run `sudo gitlab-ctl reconfigure` to compile the configuration.
 
+The next step is to tell all the other nodes where the monitoring node is:
+
+1. Edit `/etc/gitlab/gitlab.rb`, and add, or find and uncomment the following line:
+
+   ```ruby
+   gitlab_rails['prometheus_address'] = '10.0.0.1:9090'
+   ```
+
+   Where `10.0.0.1:9090` is the IP address and port of the Prometheus node.
+
+1. Save the file and [reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to
+   take effect.
+
 ## Migrating to Service Discovery
 
 Once monitoring using Service Discovery is enabled with `consul['monitoring_service_discovery'] =  true`,

doc/administration/monitoring/prometheus/index.md

@@ -145,6 +145,12 @@ To use an external Prometheus server:
    gitlab_rails['monitoring_whitelist'] = ['127.0.0.0/8', '192.168.0.1']
    ```
 
+1. On **all** GitLab Rails(Puma/Unicorn, Sidekiq) servers, set the Prometheus server IP address and listen port. For example:
+
+   ```ruby
+   gitlab_rails['prometheus_address'] = '192.168.0.1:9090'
+   ```
+
 1. To scrape NGINX metrics, you'll also need to configure NGINX to allow the Prometheus server
    IP. For example: