code: @SuppressWarnings("unchecked")
    public C downstreamConsumerFactory(Supplier<Consumer<String>> downstreamConsumerFactory) {
      this.downstreamConsumerFactory = requireNonNull(downstreamConsumerFactory);
      return (C) this;
    }
docs: Sets a downstream consumer's factory to this builder object.
  
  @param downstreamConsumerFactory A supplier of a down-stream consumer.
  @return This object