You use a multiple step Cloud Build pipeline to build and deploy your application to...

Add logic to each Cloud Build step to HTTP POST the build information to a webhook.

Add a new step at the end of the pipeline in Cloud Build to HTTP POST the build information to a webhook.

Use Stackdriver Logging to create a logs-based metric from the Cloud Buitd logs. Create an Alert with a Webhook notification type.

Create a Cloud Pub/Sub push subscription to the Cloud Build cloud-builds PubSub topic to HTTP POST the build information to a webhook.