total sessions metric

Yep, we should probably be using ``inc`` for total unique users too (sorry for not spotting that earlier).

All the metric aggregation functions happen over time bucket intervals (configurable, but 10s by default). You can find all of the aggregation function implementations at https://github.com/praekelt/vumi/blob/develop/vumi/blinkenlights/metrics.py#L201-L206.

You'll notice that ``inc`` isn't one of the aggregate types. ``inc`` is implemented manually using a counter in Redis and fires a ``last`` metric with the most recent value. See https://github.com/praekelt/vumi-jssandbox-toolkit/blob/master/lib/metrics/api.js#L103.