DOCSP-47157 doc for Ops Manager Prometheus /metrics endpoint (#836)

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- [DOCSP-47157](https://jira.mongodb.org/browse/DOCSP-47157)
-
[STAGING](https://deploy-preview-836--docs-ops-manager.netlify.app/reference/configuration/#prometheus)

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---------

Co-authored-by: John Williams <[email protected]>

Author: kanchana-mongodb

source/includes/setting-fileConf-mms.prom.listening.enabled.rst

@@ -0,0 +1,7 @@
+.. setting:: prom.listening.enabled
+
+   *Type*: boolean
+   
+   Flag that specifies whether to enable access for Prometheus. To
+   enable |mms| Prometheus endpoint, set value to ``true``. If omitted,
+   defaults to ``false``. 

source/includes/setting-fileConf-mms.prom.listening.port.rst

@@ -0,0 +1,11 @@
+.. setting:: prom.listening.port
+
+   *Type*: number
+   
+   The port over which |mms| listens for requests to the Prometheus
+   endpoint. The port must be different than the port |mms| is already
+   listening on, 8080 for |http| or 8443 for |https|. Every server will
+   listen on the specified port. So, ensure that the port is available
+   as an open port on every server. If you want to specify different
+   port for each server, use ``conf-mms.properties`` instead to specify
+   the ports. 

source/reference/configuration.txt

@@ -252,6 +252,48 @@ assignments.
 
 .. include:: /includes/setting-fileConf-mms.publicApi.accesslistEnabled.rst
 
+Prometheus 
+----------
+
+You must configure the following settings to enable the metrics that are 
+available for ingestion through Prometheus. You can also configure these
+settings in the :guilabel:`Custom` tab of the :ref:`Ops Manager Config
+<admin-console-general-om-config>` page in the
+:ref:`admin-console-general-tab`. 
+   
+After configuring these settings, you must restart |mms| on each
+instance. After restarting, the new Prometheus endpoint can be
+accessed at a |url| in the following format: 
+   
+.. code-block:: shell  
+   
+   http(s)://<opsManagerUri>:<port>/metrics
+
+.. note::
+
+   You can designate a listening port for the Prometheus metrics. This
+   doesn't have to be the default port that |mms| uses, 8080 for |http| or
+   8443 for |https|. Ensure that your network allows traffic to the
+   specified Prometheus metrics port. 
+
+After enabling, you must configure a new scrape config similar to the
+following in Prometheus for each |onprem| server that you would like to
+gather statistics from. 
+
+.. code-block:: shell 
+   :caption: scrape_config
+
+   scrape_configs:
+
+   - job_name: 'opsmanager'
+      static_configs:
+         - targets: [<opsManagerHostname>:<port>]
+
+Note that each server's endpoint only provides metrics for the same server.
+
+.. include:: /includes/setting-fileConf-mms.prom.listening.enabled.rst
+.. include:: /includes/setting-fileConf-mms.prom.listening.port.rst
+
 Push Live Migrations
 --------------------