Add pod IP to kubernetes metadata

[jsoriano]

Describe the enhancement:

Add pod IP when enriching events in kubernetes metadata. This IP can be obtained from the Status.PodIP field in a pod object, it is exposed in autodiscover as data.host, but not included in events metadata.

On a first thought, this field could be added as kubernetes.pod.ip. But actually it could be also stored in some ECS-ish field as container.ip. Update: use kubernetes.pod.ip by now for this information.

Describe a specific use case for the enhancement or feature:

Enrich log events with network information of the producing pod.

Requested in https://discuss.elastic.co/t/how-to-add-podip-on-add-kubernetes-metadata-processor/269006/5

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)