Start the raylet monitor.

def start_raylet_monitor(self):
        """Start the raylet monitor."""
        stdout_file, stderr_file = self.new_log_files("raylet_monitor")
        process_info = ray.services.start_raylet_monitor(
            self._redis_address,
            stdout_file=stdout_file,
            stderr_file=stderr_file,
            redis_password=self._ray_params.redis_password,
            config=self._config)
        assert (ray_constants.PROCESS_TYPE_RAYLET_MONITOR not in
                self.all_processes)
        self.all_processes[ray_constants.PROCESS_TYPE_RAYLET_MONITOR] = [
            process_info
        ]