First, ask if you really need "massive" scale. Is this an idea, or a well-defined product? I'd imagine if you knew what you were building, you wouldn't be here

First, ask if you really need "massive" scale. Is this an idea, or a well-defined product? I'd imagine if you knew what you were building, you wouldn't be here asking. So "massive" -- why not prototype on Postgres, and then migrate when you actually have projections on size. Different orders of magnitude change the technology you work with. Additionally, the latency with which you need to access the metrics (real time, report based). Cassandra is a pretty solid choice, Influx is really new to the game but is promising. Druid is trusted by a lot of people, Metamarkets (the author) among them, but may or may not be what you need. I'd spend some time talking to the people in #druid-dev on Freenode, they're friendly and can help guide you.