To authenticate with the Stackdriver API, you need to create a Google Cloud Platform (GCP) Service Account for the Project you want to show data for. A Grafana datasource integrates with one GCP Project. If you want to visualize data from multiple GCP Projects then you need to create one datasource per GCP Project.
The Monitoring Viewer role provides all the permissions that Grafana needs.
The following APIs need to be enabled on GCP for the datasource to work:
Detailed instructions on how to create a Service Account can be found in the documentation.
Do not forget to save your changes after uploading a file.