monitoring: update README to use new gcloud ADC command
monitoring: update README to use new gcloud ADC command

Change-Id: I160ce0bb646944dc211dfa471afd62fb54864dbe

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