I was looking on stackdriver dashboard and I found the following http request:
https://app.google.stackdriver.com/api/alerting/violation?project={project-id}&page=0&pageSize=8&policyId={policy-id}
But I didn't found any docs about this D:
The alerting methods for Stackdriver Monitoring appear in the the Google Cloud Platform documentation.
There does not seem to be an endpoint to list triggered alerts at the moment.
The best option for now would be to add a webhook as a notification channel and store the data in whatever way is easier for you to recover later.