When reviewing the documentation here (https://cloud.google.com/monitoring/alerts/incidents-events#incident) and using the general product, it appears that the open incidents and associated details are only displayed via the Google Cloud Console.
The CLI and API appear to only support management of alerting policies, but I cannot find a way to retrieve a list of open incidents. For example, I'd like to send an alert if more than 5 incidents are open for 12 hours on a specific alerting policy. It would seem that the data exists (as provided from the Google Cloud Console), but the API is not public.