If I'm not wrong, we start to access that around 1y but I think we really spent 7-10m on it.
You can send metrics from Cassandra to graphite http://www.datastax.com/dev/blog/pluggable-metrics-reporting...
Since the streaming is similar to HTTP page flow, it's not that hard.
Ex: http://blazemeter.com/blog/how-load-test-http-live-media-str...