monitoring/custommetric: simplify struct literal syntax
monitoring/custommetric: simplify struct literal syntax

Change-Id: I600204449a80ff33382a6c257ea2ef4faf4babd3

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