Slow Ops Counters

Don't think we do this, and didn't see an existing request for it...

Would it be possible to add a kv_slow_ops metric (or perhaps kv_ops_slow to match kv_ops_failed), to count per bucket / per op type, whenever a "Slow Command" log message is logged in memcached.log? (i.e. we increment the counter whenever an operation exceeds the currently configured threshold for slow ops logging for that Op type).