message: Actually fix issubclass error in metrics_serialization_test.py.
  I tried to fix this in but missed an issubclass call. This time, I actually tested the fix works on Windows.
diff: @@ -220,7 +220,7 @@ class MetricsSerialization(keras_parameterized.TestCase):
  def get_instance(x):
  if isinstance(x, str):
  return x
  - if issubclass(x, metrics.Metric):
  + if isinstance(x, type) and issubclass(x, metrics.Metric):
  return x()
  return x
  