Our architecture and the sheer number of microservices contributes to much of the scale. In order to achieve the engineering velocity and reliability goals we felt the explosion of instances with this architecture was worth it. If you consider the number of microservice instances serving user traffic (and include persistency tiers such as memcache and cassandra) you would still be in the tens of thousands.
I used carbon-relay in one job, and if you're using that, i'd guess you have 1000 machines serving users, and 30000 collecting metrics!