Marathon SD: Add port def/mapping labels (#774)

These labels were added in:
https://github.com/prometheus/prometheus/pull/2506

content/docs/operating/configuration.md

@@ -717,6 +717,8 @@ The following meta labels are available on targets during [relabeling](#relabel_
 * `__meta_marathon_image`: the name of the Docker image used (if available)
 * `__meta_marathon_task`: the ID of the Mesos task
 * `__meta_marathon_app_label_<labelname>`: any Marathon labels attached to the app
+* `__meta_marathon_port_definition_label_<labelname>`: the port definition labels
+* `__meta_marathon_port_mapping_label_<labelname>`: the port mapping labels
 
 See below for the configuration options for Marathon discovery: