private <O> PipelineBuilder<I, O> addStage(
      final Processor<T, O> processor, final String stageName) {
    return addStage(processor, bufferSize, stageName);
  }
End the pipeline with a {@link Consumer} that is the last stage of the pipeline.

@param stageName the name of this stage. Used as the label for the output count metric.
@param completer the {@link Consumer} that accepts the final output of the pipeline.
@return the constructed pipeline ready to execute.
services/pipeline/src/main/java/org/hyperledger/besu/services/pipeline/PipelineBuilder.java