Create a new metric for metricbeat

Hi there!
I'm new in the forum and even to elasticsearch and kibana.
For an university project I have to monitor several docker container created with marathon.
Initially I've used zabbix to get the docker metrics for any container but with this I can’t create charming and powerfull dashboard therefore now I'm using kibana and elastic.
I've used the metricbeat to fetch the metrics and it's worked but I need to fetch an metric called marathon_app_id beacause the container created by marathon are named mesos-13i4cxje34….. that is very hard to detect the container.
Someone can tell me how can I create a custom metric to fetch the marathon app id for metric beat
The marathon app id is contained inside the docker inspect's output.