code: func (b *Broker) OffsetCoordinator(conf OffsetCoordinatorConf) (OffsetCoordinator, error) {
  	conn, err := b.muCoordinatorConnection(conf.ConsumerGroup)
  	if err != nil {
  		return nil, err
  	}
  	if conf.Logger == nil {
  		conf.Logger = b.conf.Logger
  	}
  	c := &offsetCoordinator{
  		broker: b,
  		conf:   conf,
  		conn:   conn,
  	}
  	return c, nil
  }
docs: // OffsetCoordinator returns offset management coordinator for single consumer
  // group, bound to broker.