What should the schema look like for an API-based SaaS product?

I'm building a developer tool product, which will be accessible solely by APIs. Following are a few features I've identified that will be needed: 1. Issuing/refreshing API keys 2. Purchasing API credits 3. Subscriptions for API credits 4. Monitoring usage of API for each user What should the SCHEMA look like? Are there any open examples of schemas for such a product? Is there a term for this? Note that this would be quite similar to what something like Stripe or any API-based SaaS is doing. Just looking for a good schema example only. Any ideas?