Wouldn't you use https://cloud.google.com/armor/docs/monitoring and the Rest api (most, if not all of the Sentinel Solutions for Google leverage the api's). Caveat - I'm not a Google expert!
You can then use a custom or codeless connector to bring it into Sentinel https://learn.microsoft.com/en-us/azure/sentinel/create-custom-connector (assuming the data you need is exposed via this api).