Please review and confirm the following.
- Make sure that the Cloud API is enabled. This must be enabled in order for Mackerel to get metrics
- Make sure that the service account to be used with Google Cloud Integration does not have global permissions or those higher than viewer.
- Make sure that the service account has been assigned a role that is compatible with the service you are trying to integrate.
- If you have specified a label in "Filter by label", make sure that the label has been correctly assigned to the target Google Cloud resource as well.