Monitor execution and the workflow runner are deliberately unguarded: billing
state must not take away a customer's ability to know their infrastructure is
on fire, and killing a run midway leaves a half-configured server.
The plan called for a server-limit check in gRPC Register. Left out: a server
row only comes from CreateServer, which already checks the cap, so counting in
Register counts the caller itself and would reject a legitimate agent at
exactly the cap.