code: def start_policy_update_timer(self, number_experiences: int, mean_return: float):
          
          self.last_buffer_length = number_experiences
          self.last_mean_return = mean_return
          self.time_policy_update_start = time()
docs: Inform Metrics class that policy update has started.
          :int number_experiences: Number of experiences in Buffer at this point.
          :float mean_return: Return averaged across all cumulative returns since last policy update