code: func projectID() string {
  	if *gceProjectID != "" {
  		return *gceProjectID
  	}
  	projID, err := metadata.ProjectID()
  	if projID == "" || err != nil {
  		log.Fatalf("GCE project ID needed but --gce_project_id not specified (and not running on GCE); metadata error: %v", err)
  	}
  	return projID
  }
docs: // projectID returns the GCE project ID used for running this camweb on GCE
  // and/or for logging on Google Cloud Logging, if any.