    def start(self):
        """
        Start the periodic update process
        """
        if self.active_users_enabled:
            # Setup periodic refresh of the metric
            self._periodic_callbacks["active_users"] = PeriodicCallback(
                self.update_active_users,
                self.active_users_update_interval * 1000,
                jitter=0.01,
            )

            # Update the metrics once on startup too
            self.update_active_users()

        if self.event_loop_interval_enabled:
            self._tasks["event_loop_tick"] = asyncio.create_task(
                self._measure_event_loop_interval()
            )

        # start callbacks
        for pc in self._periodic_callbacks.values():
            pc.start()
Start the periodic update process
jupyterhub/metrics.py