title: Dataflow writing a pCollection of GenericRecords to Parquet files
body: <p>In apache beam step I have a PCollection of <code>KV&lt;String, Iterable&lt;KV&lt;Long, GenericRecord&gt;&gt;&gt;&gt;</code>.
  I want to write all the records in the iterable to the same parquet file. My code snippet is given below</p>
  
  <pre><code>p.apply(ParDo.of(new MapWithAvroSchemaAndConvertToGenericRecord())) // PCollection&lt;GenericRecord&gt;
  .apply(ParDo.of(new MapKafkaGenericRecordValue(formatter, options.getFileNameDelimiter()))) //PCollection&lt;KV&lt;String, KV&lt;Long, GenericRecord&gt;&gt;&gt;
  .apply(GroupByKey.create()) //PCollection&lt;KV&lt;String, Iterable&lt;KV&lt;Long, GenericRecord&gt;&gt;&gt;&gt;&gt;
  </code></pre>
  
  <p>now I want to write all the Records in the Iterable in the same parquet file(derive the file name by the key of KV).</p>