What opensource solutions can be used to implement anomaly detection based on Riemann metrics?
I can think of 3 possibility (Not exactly Riemann metrics based, but around anomaly detection):


Outdated but the most efficient in anomaly detectionI know: Shyline + Oculus from Etsy Both tools available from etsy's github as Skyline and oculus, check the network graph for more up to date forks. Best one I found is earthgecko/skyline with its doc
Graphite and checking that the Standard deviation fits in the Holtz Winter's Confidence bands
ELK stack in version 5 with timelion which, as above,  allow to compute a standard deviation and alert if it goes out of confidence bands with the proper license.