code: func getUpdateMask(set *flag.FlagSet, paths map[string]string) *field_mask.FieldMask {
  	m := &field_mask.FieldMask{}
  	set.Visit(func(f *flag.Flag) {
  		if path, ok := paths[f.Name]; ok {
  			m.Paths = append(m.Paths, path)
  		}
  	})
  	sort.Strings(m.Paths)
  	return m
  }
docs: // getUpdateMask returns a *field_mask.FieldMask containing paths based on which flags have been set.