code: func AddLabelToSelector(selector *metav1.LabelSelector, labelKey, labelValue string) *metav1.LabelSelector {
  	if labelKey == "" {
  		// Don't need to add a label.
  		return selector
  	}
  	if selector.MatchLabels == nil {
  		selector.MatchLabels = make(map[string]string)
  	}
  	selector.MatchLabels[labelKey] = labelValue
  	return selector
  }
docs: // AddLabelToSelector returns a selector with the given key and value added to the given selector's MatchLabels.