code: def _merge_map(key, values, partial):
    
    proto = kv_pb.KeyValues()
    proto.set_key(key)
    proto.value_list().extend(values)
    yield proto.Encode()
docs: A map function used in merge phase.
  
    Stores (key, values) into KeyValues proto and yields its serialization.
  
    Args:
      key: values key.
      values: values themselves.
      partial: True if more values for this key will follow. False otherwise.
  
    Yields:
      The proto.