Add KPI Metrics logging statement

[bbangert]

A new metric statement should be added when the client drops its connection, that logs some session statistics for the user-session.

Example logging message:

self.log.info("Session", connection_type="websocket", messages=3, register=0,
              unregister=0, new_uaid=False, uaid_hash="...", user-agent="..."
              )

This is needed for the following KPI's:

1. Daily Activations - Knowing new_uaid for a session indicates if the uaid is a reconnect or new.
2. DIU’s - Daily Inactive Users, to know one, I'd need to see that for all the users sessions for a day, there were no messages.

It's possible to get both of these by alternatively logging when a new uaid is issued, and logging 'hello' for a uaid_hash, which would let me group them to see there were no Ack's. That's likely just as efficient, but correlating commands issued per 'session', and other per-session stats would be hard to evaluate. As such, it'd be easier to do analysis on a Session logging statement that had it all.

A side-effect of this is that long-lived connections get a bit.... weird. This shouldn't be that bad for our stats, as the max typical life span of a connection right now is around 50 minutes.