input: Write a function to calculate the maximum, minimum, and average values of a list.
  assert calculate_metrics([3, 4, 5, 6, 21, 8]) == (21, 3, 7.833333333333333)
  assert calculate_metrics([10, 20, 30, 40, 50]) == (50, 10, 30.0)
  assert calculate_metrics([1, 2, 3, 4, 5]) == (5, 1, 3.0)
output: def calculate_metrics(lst):
      maximum = max(lst)
      minimum = min(lst)
      average = sum(lst) / len(lst)
      return maximum, minimum, average