Improve configuration documentation with new Datadog contrib

This adds a few extra documentation line for the configuration to allow
user to find all the settings they can tweak for each individual
contribs instead of having to go through each individual contrib files.

doc/configuration.rst

@@ -726,6 +726,12 @@ pause_enabled
   If false, disables pause/unpause operations and hides the pause toggle from
   the visualiser.
 
+metrics_collector
+  Optional setting allowing Luigi to use a contribution to collect metrics
+  about the pipeline to a third-party. By default this uses the default metric
+  collector that acts as a shell and does nothing. The only currently available
+  option is "datadog".
+
 
 [sendgrid]
 ----------
@@ -893,6 +899,28 @@ user
   this parameter is not honored by any of the other hdfs clients, you should
   think twice before setting this parameter.
 
+[datadog]
+--------
+
+api_key
+  The api key found in the account settings of Datadog under the API
+  sections.
+app_key
+  The application key found in the account settings of Datadog under the API
+  sections.
+default_tags
+  Optional settings that adds the tag to all the metrics and events sent to
+  Datadog. Default value is "application:luigi".
+environment
+  Allows you to tweak multiple environment to differentiate between production,
+  staging or development metrics within Datadog. Default value is "development".
+statsd_host 
+  The host that has the statsd instance to allow Datadog to send statsd metric. Default value is "localhost".
+statsd_port 
+  The port on the host that allows connection to the statsd host. Defaults value is 8125.
+metric_namespace
+  Optional prefix to add to the beginning of every metric sent to Datadog.
+  Default value is "luigi".
 
 Per Task Retry-Policy
 ---------------------