code: def testAddedField(self):
      a = sc_pb.ResponseObservation()
      b = sc_pb.ResponseObservation(
          observation=sc_pb.Observation(game_loop=1)
      )
      diff = proto_diff.compute_diff(a, b)
      self.assertIsNotNone(diff)
      self.assertLen(diff.added, 1, diff)
      self.assertEqual(str(diff.added[0]), "observation")
      self.assertEqual(diff.added, diff.all_diffs())
      self.assertEqual(diff.report(), "Added observation.")
  
imports: from absl.testing import absltest
  from pysc2.lib import proto_diff
  from s2clientprotocol import sc2api_pb2
  from s2clientprotocol import score_pb2
label_desc: Use image node_id 2 for calling the ProtoDiffTest obj's underlying member method code with example usage: obj.testAddedField() without return types