monitoring/custommetric: add more sleep to tests
monitoring/custommetric: add more sleep to tests

GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,danicat/golang-samples,danicat/golang-samples,GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples