In my opinion, the best way to do it is ingesting events in a relational db with an idempotency key to prevent duplicated events. Some companies don't need real time billing data, so you could just use it to aggregate these events with the aggregation function that fits your need for a certain feature. Thus, you have the flexibility of (i) calculating pretty much everything you want, (ii) give granularity of usage to your customers and (iii) be closer to real time (you play the query anytime you need it).
