Marathon_SD Host Target Address when using Virtual Networks

[Smithx10]

Proposal

While using the Marathon_SD configuration with a prometheus server external to Mesosphere Cluster ( which uses marathon ) I ran into a situation where the Port's Target Address would use the Virtual IP instead of the HOST IP, which is fair if the Prometheus Server was internal to the DC/OS cluster.

I added an external option to the config which will bias the ports Target Address to the Host IP which is routable externally. If omitted, marathon_sd behaves the same as before. So this should be a non breaking change.

Smithx10@2780ee5

In IRC someone suggested relabelling, but I don't believe there is any meta labels that would facilitate this behaviour.

__meta_marathon_app: the name of the app (with slashes replaced by dashes)
__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
__meta_marathon_port_index: the port index number (e.g. 1 for PORT1)

https://prometheus.io/docs/prometheus/latest/configuration/configuration/#marathon_sd_config

[Smithx10]

After receiving feedback from @brian-brazil in #5068 I have added the meta_marathon_host label to the marathon_sd configuration.

This should be addressed in https://github.com/Smithx10/prometheus/tree/marathon_sd_host_md