code: def test_unknown_metric(self):
      history = trax_history.History()
      history.append("train", "metric1", 1, 0.1)
      self.assertEqual(history.get("train", "unknown_metric"), [])
  
file_path: trax/trax/supervised/history_test.py
imports: from absl.testing import absltest
  from trax.supervised import history
label_desc: Use image node_id 2 for calling the HistoryTest obj's underlying member method code with example usage: obj.test_unknown_metric() without return types