code: func MaxOutstandingMessages(max int) broker.SubscribeOption {
  	return func(o *broker.SubscribeOptions) {
  		if o.Context == nil {
  			o.Context = context.Background()
  		}
  
  		o.Context = context.WithValue(o.Context, maxOutstandingMessagesKey{}, max)
  	}
  }
docs: // MaxOutstandingMessages sets the maximum number of unprocessed messages
  // (unacknowledged but not yet expired) to receive.