exporter/stackdriver: Auto-populate k8s_container MonitoredResource

[mayurkale22]

After finishing #38, Please use that util to replace gke_container with k8s_container in Stackdriver exporter with new resource labels.

More Info:
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.

Counterpart in java: census-instrumentation/opencensus-java#1467