[prometheus] Added documentation for deployment and setting extra options

Change-Id: I8bb39eaf8a4239c37fcbf91b55ec8003542e2506

doc/source/reference/logging-and-monitoring/index.rst

@@ -11,4 +11,5 @@ logging and monitoring services available in kolla.
    central-logging-guide
    monasca-guide
    osprofiler-guide
+   prometheus-guide
    skydive-guide

doc/source/reference/logging-and-monitoring/prometheus-guide.rst

+.. _prometheus-guide:
+
+=====================================================
+Prometheus - Monitoring System & Time Series Database
+=====================================================
+
+Overview
+~~~~~~~~
+
+Kolla can deploy a full working Prometheus setup in either a **all-in-one** or
+**multinode** setup.
+
+Preparation and deployment
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To enable Prometheus, modify the configuration file ``/etc/kolla/globals.yml``
+and change the following:
+
+.. code-block:: yaml
+
+   enable_prometheus: "yes"
+
+Extending the default command line options
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is possible to extend the default command line options for Prometheus by
+using a custom variable. As an example, to set remote timeouts to 30 seconds
+and data retention period to 2 days:
+
+.. code-block:: yaml
+
+   prometheus_cmdline_extras: "-storage.remote.timeout 30s -storage.local.retention 48h"