code: func (p *Pusher) Grouping(name, value string) *Pusher {
  	if p.error == nil {
  		if !model.LabelName(name).IsValid() {
  			p.error = fmt.Errorf("grouping label has invalid name: %s", name)
  			return p
  		}
  		if strings.Contains(value, "/") {
  			p.error = fmt.Errorf("value of grouping label %s contains '/': %s", name, value)
  			return p
  		}
  		p.grouping[name] = value
  	}
  	return p
  }
docs: // Grouping adds a label pair to the grouping key of the Pusher, replacing any
  // previously added label pair with the same label name. Note that setting any
  // labels in the grouping key that are already contained in the metrics to push
  // will lead to an error.
  //
  // For convenience, this method returns a pointer to the Pusher itself.
  //
  // Note that until https://github.com/prometheus/pushgateway/issues/97 is
  // resolved, this method does not allow a “/” character in the label value.