This. Any new large query or aggregation in the Borgmon/Prometheus model requires re-solving federation, and continuing maintenance of runtime configuration. That might technically be scalable in that you could do it but you have to maintain it, and pay the labor cost. It's not practical over a certain size or system complexity. It's also friction. You can only do the queries you can afford to set up.

That's why Google spent all that money to build Monarch. At the end of the day Monarch is vastly cheaper in person time and resources than manually-configured Borgmon/Prometheus. And there is much less friction in trying new queries, etc.