Keep your disks when a VM is deleted.

gcloud compute instances set-disk-auto-delete server --no-auto-delete --disk=server-disk-1
gcloud compute instances set-disk-auto-delete server --no-auto-delete --disk=server-disk-2

To delete.

gcloud compute instances set-disk-auto-delete server --auto-delete --disk=server-disk-1
gcloud compute instances set-disk-auto-delete server --auto-delete --disk=server-disk-2