nsqd/nsqadmin: introspection of producer connections

nsqadmin does a great job at exposing the consumers on a given topic/channel but there is no way to introspect the producers connected via TCP to a nsqd node.

nsqd should expose a /producer_stats endpoint that lists the connections it has that are not subscribe to a topic. It should list the same client information available in /stats (tls, remote address, connection time, user_agent, auth, etc). It should also list a lifetime connection stat for number of messages published.

nsqadmin should have a new page (or section on the individual /node/:node page) that exposes this information.