bytes_written stat is not updated

During testing with Prometheus Timofey Barmin identified that the "bytes_written" stat stays at zero. On current master, this is also the case when exposed over CMD_STAT; a cursory search of the code suggests that nothing updates thread_stats.bytes_written.

Timofey Barmin noted that 6.5.0 is not affected.

It's possible that this is accidental or that this stat is intentionally no longer tracked.