Stop assuming zonal cluster deployment

[MontyCarter]

opencensus-go-exporter-stackdriver/stackdriver.go

Line 325 in 42e7e58

zone, err := metadataapi.Zone()

This code works when a GKE cluster is zonally deployed, but is incorrect when run against a regional cluster. With a regional cluster, the Location should be the region name, not the underlying zone name.

This needs to use metadataapi.InstanceAttributeValue("cluster-location") (similar to here: https://github.com/knative/pkg/blob/7d1b0f19ef717da9e0df0ce1e35e1ca81627445f/metrics/gcp_metadata.go#L44)

[dashpole]

You are probably right. Using the region in regional clusters would match what GKE does for container/pod/cluster metrics.