add doc about exometer reporters

doc/operation-and-maintenance/Logging-&-monitoring.md

@@ -43,5 +43,49 @@ You can read more about it here: [REST interface to metrics](/developers-guide/R
 ### Wombat OAM
 
 WombatOAM is an operations and maintenance framework for Erlang based systems. Its Web Dashboard displays this data in an aggregated manner and provides interfaces to feed the data to other OAM tools such as Graphite, Nagios or Zabbix.
+
 For more information see:
-https://www.erlang-solutions.com/products/wombat
+https://www.erlang-solutions.com/products/wombat
+
+### Built-in Exometer reporters
+
+MongooseIM uses the Exometer libary for collecting the metrics. Exometer has many
+build-in reporters that can send metrics to external services like:
+
+* graphite
+* amqp
+* statsd
+* snmp
+* opentsdb
+
+It is possible to enable them in Moongoose via  the `app.config` file. The file sits next
+to the `ejabberd.cfg` file and both files are located in the `rel/files` and `_REL_DIR_/etc` directories.
+For more details, please visit the Exometer's project page: [ExometerProject](https://github.com/Feuerlabs/exometer).
+
+**Note that we are using the 1.1 version.**
+
+Below you can find sample configuration, it setups graphite reporter which connects
+to graphite running on localhost.
+
+You can see an additional option not listed in the Exometer docs - `mongooseim_report_interval`.
+That option sets metrics resolution - in other words: how often Exometer gathers and sends metrics
+throught reporters. By default that is 60 seconds.
+
+```erl
+...
+{exometer, [
+    {mongooseim_report_interval, 60000}, %% 60 seconds
+    {report, [
+        {reporters, [
+                     {exometer_report_graphite, [
+                                                 {prefix, "mongooseim"},
+                                                 {connect_timeout, 5000},
+                                                 {host, "127.0.0.1"},
+                                                 {port, 2003},
+                                                 {api_key, ""}
+                                                ]}
+                    ]}
+    ]}
+  ]}
+...
+```