code: func Label(k, v string) selector.Option {
  	return func(o *selector.Options) {
  		l, ok := o.Context.Value(labelKey{}).([]label)
  		if !ok {
  			l = []label{}
  		}
  		l = append(l, label{k, v})
  		o.Context = context.WithValue(o.Context, labelKey{}, l)
  	}
  }
docs: // Label used in the priority label list