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.
(I've learned not to trust numbers that seem too good to be true unless they're contractually obligated.)