Add image that shows metricsets within a module (elastic#2911)

* Add image that shows metricsets within a module

metricbeat/docs/how-metricbeat-works.asciidoc

@@ -12,15 +12,14 @@ separate event, metricsets retrieve a list of multiple related metrics in a sing
 to the remote system. So, for example, the Redis module provides an `info`
 metricset that collects information and statistics from Redis by running the
 http://redis.io/commands/INFO[`INFO`] command and parsing the returned result.
+
+image:./images/module-overview.png[Modules and metricsets]
+
 Likewise, the MySQL module provides a `status` metricset that collects data
 from MySQL by running a http://dev.mysql.com/doc/refman/5.7/en/show-status.html[`SHOW GLOBAL STATUS`]
-SQL query.
-
-Metricsets make it easier for you by grouping sets of related metrics together
+SQL query. Metricsets make it easier for you by grouping sets of related metrics together
 in a single request returned by the remote server.
 
-//TODO: ADD DIAGRAM HERE THAT SHOWS A COUPLE OF MODULES AND METRICSETS WITHIN THOSE MODULES.
-
 Metricbeat retrieves metrics by periodically interrogating the host system based
 on the `period` value that you specify when you configure the module. Because multiple
 metricsets can send requests to the same service, Metricbeat reuses connections

metricbeat/docs/overview.asciidoc

@@ -14,6 +14,7 @@ and the following services:
   * <<metricbeat-module-mongodb,MongoDB>>
   * <<metricbeat-module-mysql,MySQL>>
   * <<metricbeat-module-nginx,Nginx>>
+  * <<metricbeat-module-postgresql,PostgreSQL>>
   * <<metricbeat-module-redis,Redis>>
   * <<metricbeat-module-system,System>>
   * <<metricbeat-module-zookeeper,Zookeeper>>