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

def start_policy_update_timer(self, number_experiences: int, mean_return: float):
        """
        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
        """
        self.last_buffer_length = number_experiences
        self.last_mean_return = mean_return
        self.time_policy_update_start = time()