Extend metrics middleware

This should allow us to track via Graphite, for each transport, network operator and tag or tagpool:

* number of sessions
* number of messages
* time taken for each session
* time taken for each session, rounded to the next ``N`` seconds (this is to allow very rough session costs calculations since some networks bill per ``N`` seconds or part there-of).

See also praekelt/vumi#856.
Should we maybe rename this to `provider_metrics`? That's the field we're using in the message object and it's more inclusive of things like twitter and mxit than "network operator".