need counter stats for ep-engine public operations such as touch, gat, observe

Earlier today I had the occasion to help a user who was trying to determine where the latency is in their code which is updating some documents and then checking the state of those via the observe command on the replicas.

To diagnose it, ideally there would have been a cmd_observe stat counter much like we have for all other public operations in memcached protocol. Unfortunately, there is not.

This request is to add stats for all extended commands in ep_engine, especially the public ones.