code: def test_variable_tracking(self):
      # In list
      metric = ExampleMetric(name="mse")
      metric.more_vars = [backend.Variable(0.0), backend.Variable(1.0)]
      self.assertEqual(len(metric.variables), 4)
  
      # In dict
      metric = ExampleMetric(name="mse")
      metric.more_vars = {
          "a": backend.Variable(0.0),
          "b": backend.Variable(1.0),
      }
      self.assertEqual(len(metric.variables), 4)
  
      # In nested structured
      metric = ExampleMetric(name="mse")
      metric.more_vars = {
          "a": [backend.Variable(0.0), backend.Variable(1.0)]
      }
      self.assertEqual(len(metric.variables), 4)
  
imports: import numpy
  from keras import backend
  from keras import initializers
  from keras import metrics
  from keras import ops
  from keras import testing
  from keras.metrics.metric import Metric
label_desc: Use image node_id 4 for calling the MetricTest obj's underlying member method code with example usage: obj.test_variable_tracking() without return types