code: func (g *Cloud) ListInstanceNames(project, zone string) (string, error) {
  	ctx, cancel := cloud.ContextWithCallTimeout()
  	defer cancel()
  
  	l, err := g.c.Instances().List(ctx, zone, filter.None)
  	if err != nil {
  		return "", err
  	}
  	var names []string
  	for _, i := range l {
  		names = append(names, i.Name)
  	}
  	return strings.Join(names, " "), nil
  }
docs: // ListInstanceNames returns a string of instance names separated by spaces.
  // This method should only be used for e2e testing.
  // TODO: remove this method.