Limit what a user can consume

This feature costs me money per use: <describe> and I need limits.

- Where the counter lives, and how it handles two simultaneous
  requests without letting both through.
- The reset boundary — rolling window or calendar month? Say which
  and why.
- What the user sees as they approach and hit the limit. The message
  must say when it resets.
- An overall cap across all users, independent of per-user limits, so
  one bug can't run up the bill.
- How I raise a specific user's limit without a deploy.

Then tell me what happens today if someone scripts this endpoint.