Monitored Resource: replace gke_container with k8s_container in Stackdriver.

[mayurkale22]

The names of the monitored resource types used by Kubernetes have changed. Link: https://cloud.google.com/monitoring/kubernetes-engine/migration#resource_type_changes

Current (old) release type: gke_container, Beta (new) release type: k8s_container

Resource label changes: The labels in the new monitored resource types are slightly different from the labels used in the current support. Examples: namespace_id -> namespace_name, pod_id -> pod_name and zone -> location.

The labels in the new monitored resource types are slightly different from the labels used in the older types. The set of resource labels for k8s_container: cluster_name, container_name, namespace_name, location, pod_name (no instance_id).

/cc @fabxc @bogdandrutu