From my look at Riemann, it seems more aimed as an event monitoring system than a time-series monitoring system. You can (and many do) use Riemann as time-series monitoring system, my understanding is that Prometheus is a bit better for multi-dimensional labels.

I could see Riemann being used as an alert manager on top of Prometheus, handling all the logic around de-duping of alerts and notification. Prometheus's own alert manager is considered experimental.