The Google Cloud Pub/Sub API exports usage metrics that can be monitored programmatically or accessed via Stackdriver Monitoring. You can create dashboards, set up alerts, and set up an autoscaler to manage publisher or message processing (subscriber) instances running on Compute Engine.
Note that although Google Cloud Pub/Sub itself is scaled automatically, you are responsible for managing your Pub/Sub quota.
Go to Stackdriver in Google Cloud Platform Console to view Stackdriver monitoring dashboards or to define Stackdriver alerts. You can also use the Stackdriver monitoring API to query and view metrics for subscriptions and topics.