Default Value Type for prometheus-to-sd

The current default type is ["INT64"](https://github.com/GoogleCloudPlatform/k8s-stackdriver/blob/master/prometheus-to-sd/translator/translator.go#L282)

This is breaking floating point metrics such as: "custom.googleapis.com/kube-state-metrics/kube_pod_container_resource_requests_cpu_cores"

Since the variable that holds the Value is of type float64 already, I think the best candidate for default type should be "DOUBLE" instead of "INT64".