Update external prometheus config

Updated to include missing metrics:
    * GitLab Workhorse
    * Nginx
    * Sidekiq

doc/administration/monitoring/prometheus/index.md

@@ -98,11 +98,13 @@ To use an external Prometheus server:
 
     ```ruby
      gitlab_monitor['listen_address'] = '0.0.0.0'
+     sidekiq['listen_address'] = '0.0.0.0'
      gitlab_monitor['listen_port'] = '9168'
-     gitaly['prometheus_listen_addr'] = "0.0.0.0:9236"
      node_exporter['listen_address'] = '0.0.0.0:9100'
      redis_exporter['listen_address'] = '0.0.0.0:9121'
      postgres_exporter['listen_address'] = '0.0.0.0:9187'
+     gitaly['prometheus_listen_addr'] = "0.0.0.0:9236"
+     gitlab_workhorse['prometheus_listen_addr'] = "0.0.0.0:9229"
      ```
 
 1. Install and set up a dedicated Prometheus instance, if necessary, using the [official installation instructions](https://prometheus.io/docs/prometheus/latest/installation/).
@@ -112,6 +114,18 @@ To use an external Prometheus server:
     gitlab_rails['monitoring_whitelist'] = ['127.0.0.0/8', '192.168.0.1']
     ```
 
+1. To scrape nginx metrics, you'll also need to configure nginx to allow the Prometheus server
+   IP. For example:
+
+    ```ruby
+    nginx['status']['options'] = {
+          "server_tokens" => "off",
+          "access_log" => "off",
+          "allow" => "192.168.0.1",
+          "deny" => "all",
+    }
+    ```
+
 1. [Reconfigure GitLab][reconfigure] to apply the changes
 1. Edit the Prometheus server's configuration file.
 1. Add each node's exporters to the Prometheus server's