code: def testSingleElement(self):
      with tf.Session() as sess:
          with tempfile.NamedTemporaryFile(
              mode="w+t", delete=False
          ) as vocab:
              vocab.writelines([word + "\n" for word in self.vocab])
              vocab.flush()
              preprocessing_fn = utils.metrics_preprocessing_fn(
                  vocab.name, "text_a", "lang"
              )
              outputs = preprocessing_fn(self.raw_data)
              tf.tables_initializer().run()
              outputs = sess.run(outputs)
  
              self.assertEqual(outputs["lang"], "en")
              self.assertEqual(outputs["num_non_unk_wordpieces"], 7)
              self.assertEqual(outputs["num_preserved_chars"], 20)
              self.assertEqual(outputs["num_dropped_chars"], 3)
              self.assertSequenceAlmostEqual(
                  outputs["wordpieces"].values, self.expected_wordpieces
              )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import collections
  import logging
  import tempfile
  from absl.testing import absltest
  import apache_beam
  from apache_beam.testing.test_pipeline import TestPipeline
  from apache_beam.testing.util import assert_that
  from apache_beam.testing.util import equal_to
  import tensorflow.compat.v1
  from tensorflow_text.tools.wordpiece_vocab import utils
label_desc: Use image node_id 2 for calling the MetricsPreprocessingFnTest obj's underlying member method code with example usage: obj.testSingleElement() without return types