如何在beam FileIO.matchAll()结果中添加附加字段？

import apache_beam as beam

class AddAdditionalFieldFn(beam.DoFn):
    def process(self, element):
        # 添加附加字段"additional_field"
        element['additional_field'] = 'additional_value'
        yield element

# 创建一个Pipeline
p = beam.Pipeline()

# 从文件模式匹配的所有文件中读取数据
files = p | 'Match Files' >> beam.io.fileio.MatchAll(file_pattern)

# 读取匹配文件的内容
lines = files | 'Read Files' >> beam.io.fileio.ReadMatches()

# 应用自定义的DoFn函数来添加附加字段
result = lines | 'Add Additional Field' >> beam.ParDo(AddAdditionalFieldFn())

# 打印结果
result | 'Print Output' >> beam.Map(print)

# 运行Pipeline
p.run()