Metering, pricing, and billing is way more complicated than you might assume. There are historical posts here with more details if you search. In short its all async, theres variable lag, theres multiple “types” or dimensions to metering, the prices vary by SKU + customer + previous metering or billing value + other SKU usage, and billing is not uniform across customers. Imagine needing to accumulate all the metering events, apply pricing plans, modify price based on accumulated value, then periodically recalculate it all to compute a bill, then apply more transforms and credits, and that gets to the billable price.

Now evaluate your “cap” rules (which will be just as complex) and feed that back to the actual admin/control plane of the service.