I'm a happy customer of Wavefront. I completely believe that Datadog can handle hundreds of thousands per minute -- especially if most of them are pre-canned, non-custom metrics grabbed by their agent. Hundreds of thousands of metrics/minute is a few thousand a second only. Wavefront does millions of custom metrics per second, which can be sent with different dimensions and tags. That's much harder.
Prometheus can do 800k/s on a single machine. Handling a million per second sounds perfectly plausible to me if you design it properly. The question is more how much it's going to cost you.
Wavefront doesn't publish pricing, but if we take Librato's pricing as a general indication you're talking several million dollars a month.
