The supported resource types that can be used as values of
group_resource.resource_type. INSTANCE includes
gce_instance and aws_ec2_instance resource types. The
resource types gae_app and uptime_url are not valid here
because group checks on App Engine modules and URLs are not allowed.
Values:
RESOURCE_TYPE_UNSPECIFIED (0):
Default value (not valid).
INSTANCE (1):
A group of instances from Google Cloud
Platform (GCP) or Amazon Web Services (AWS).
AWS_ELB_LOAD_BALANCER (2):
A group of Amazon ELB load balancers.
