Avoid resolving monitored resource every exported timeseries.

[bogdandrutu]

Few more fixes:

1. Use gke_container as resource type instead of k8s_container when provided as gke_container.
2. Add support for k8s_container when provided in the options
3. Use provided default_monitored_labels if type is one of gce_instance, gke_container, aws_ec2_instance.

[bogdandrutu]

_default_labels is for metric labels not for resource labels.

[c24t]

After talking to @bogdandrutu we'll wait on this and #426 until adding support for resources.