Your question is Design a System Under a Request Cap. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You are given a service that can take a maximum of 1B requests. If you exceed that count, you will get billed. How would you design a system to ensure you do not exceed the cap?