Hi… The way to do that on Fly.io is retroactive:
No caps or circuit-breakers can be set a priori, and Fly seems very against ever having such a feature (except in a hypothetical future students-only tier).
They consider the idea of wanting a site to be simply turned off to place a person outside of the professional-minded clientele that they’re primarily catering to, basically.
(I don’t fully agree with this myself, but I at least see where they’re coming from…)
Hope this helps!
Aside: Credits are actually worse for this, since they’re consumed immediately—with no waiver.