The filter for resource.type="k8s_cluster" contains Audit Logs for System Events that are always enabled, hence, they won't be excluded from Stackdriver Logging unlike resource.type="container" that only has container stdout and stderr logging.
Furthermore, you can determine which logs belong to the Audit Logs stream by adding the '@type' condition to your Stackdriver filter:
resource.type="k8s_cluster"
protoPayload.serviceName="k8s.io"
protoPayload.resourceName="app.k8s.io/v1beta1/namespaces/default/applications/prometheus-1"
protoPayload."@type"="type.googleapis.com/google.cloud.audit.AuditLog"
Finally, Grafana is able to display these log-based metrics since they're exported into it via Datasource instead of going into Stackdriver.