Extend metrics middleware

This should allow us to track via Graphite:

* number of sessions per transport per day
* number of messages per transport per day
* number of sessions per to_addr per day (maybe a specific set)
* number of messages per to_addr per day (maybe a specific set)
* number of sessions per operator (where operator is defined as a set of from_addr prefixes)
* number of messages per operator (where operator is defined as a set of from_addr prefixes)

See also praekelt/vumi#856.