code: func (c *ComputeUtil) addFirewallTag(instance *raw.Instance) error {
  	log.Infof("Adding tag for the firewall rule")
  
  	tags := instance.Tags
  	for _, tag := range tags.Items {
  		if tag == firewallTargetTag {
  			return nil
  		}
  	}
  
  	tags.Items = append(tags.Items, firewallTargetTag)
  
  	op, err := c.service.Instances.SetTags(c.project, c.zone, instance.Name, tags).Do()
  	if err != nil {
  		return err
  	}
  
  	return c.waitForRegionalOp(op.Name)
  }
docs: // addFirewallTag adds a tag to the instance to match the firewall rule.