Change gcm3 to use google.cloud.monitoring library

[MartinMikita]

Google Custom Metric (using API v3) gcm3 need to be updated for correct use in Scaling Based on Stackdriver Monitoring Custom Metrics.

Related documents:

• https://cloud.google.com/compute/docs/autoscaler/scaling-stackdriver-monitoring-metrics
• https://cloud.google.com/monitoring/custom-metrics/creating-metrics#defining_your_metric
• https://cloud.google.com/monitoring/docs/reference/libraries#client-libraries-install-python
• https://github.com/GoogleCloudPlatform/google-cloud-python/tree/master/monitoring/google/cloud/monitoring
• https://googlecloudplatform.github.io/google-cloud-python/latest/monitoring/usage.html#writing-custom-metrics