How do you manage subscriptions do usage based billing?

Hello HN Community. Till now I was using Stripe for Pinggy.io . But Stripe does not allow me to track usage ( such as how much GB of data is used ) per subscription.

This is important becasuse with each subscription seat I want to provide X GB of free data transfer. Then charge something for the rest. But stripe does not allow me to track the usage on a per subscription granularity.

I could not find any decent open source alternative. Currently I am exploring KillBill, but seems like I have to reqrite the Stripe plugin for that.

How do you manage your subscriptions and usage based billing? I know that for AI startups, this is a must.