code: func GroupIDFunction(groupfunc StringFromMessageFunc) broker.Option {
  	return func(o *broker.Options) {
  		if o.Context == nil {
  			o.Context = context.Background()
  		}
  		o.Context = context.WithValue(o.Context, groupIdFunctionKey{}, groupfunc)
  	}
  }
docs: // GroupIDFunction sets the function used to create the group ID string for a
  // given message