<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def health(self):
 """Health check method, returns the up-time of the device."""

        return json.dumps(dict(uptime='{:.3f}s'
                               .format((time.time() - self._start_time))))