code: func AllPreferredGroupVersions() string {
  	if len(groupMetaMap) == 0 {
  		return ""
  	}
  	var defaults []string
  	for _, groupMeta := range groupMetaMap {
  		defaults = append(defaults, groupMeta.GroupVersion.String())
  	}
  	sort.Strings(defaults)
  	return strings.Join(defaults, ",")
  }
docs: // AllPreferredGroupVersions returns the preferred versions of all registered
  // groups in the form of "group1/version1,group2/version2,..."