code: @Override
    public GoogleCloudStorageFileSet finish(Collection<? extends OutputWriter<ByteBuffer>> writers)
        throws IOException {
      List<String> fileNames = new ArrayList<>();
      for (OutputWriter<ByteBuffer> writer : writers) {
        SizeSegmentingGoogleCloudStorageFileWriter segWriter =
            (SizeSegmentingGoogleCloudStorageFileWriter) writer;
        for (GoogleCloudStorageFileOutputWriter delegatedWriter : segWriter.getDelegatedWriters()) {
          fileNames.add(delegatedWriter.getFile().getObjectName());
        }
      }
      return new GoogleCloudStorageFileSet(bucket, fileNames);
    }
docs: Returns a list of all the filenames written by the output writers
  
  @throws IOException