monitoring: make custom metric test more resilient (#673)
monitoring: make custom metric test more resilient (#673)

Custom metric test has been wrapped with retry handlers for some
methods, but not all.  In particular, recent test failures are
related to HTTP 500 backend errors for writing timeseries.
GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples,GoogleCloudPlatform/golang-samples