Incorrect value for `location` in GKE for regional GKE clusters.

[gkevinzheng]

See googleapis/google-cloud-go#9083

In google/cloud/logging_v2/handlers/_monitored_resources.py, we use the value of zone for the value of the location label in GKE:

python-logging/google/cloud/logging_v2/handlers/_monitored_resources.py

Lines 97 to 109 in 6d265bf

	zone = retrieve_metadata_server(_ZONE_ID)
	cluster_name = retrieve_metadata_server(_GKE_CLUSTER_NAME)
	project = retrieve_metadata_server(_PROJECT_NAME)
	resource = Resource(
	type="k8s_container",
	labels={
	"project_id": project if project else "",
	"location": zone if zone else "",
	"cluster_name": cluster_name if cluster_name else "",
	},
	)
	return resource

This is incorrect for regional GKE clusters, because the value of location should be the region where it is deployed, and is inconsistent with the value of location for other logging statements.