Ask HN: API as a Service?

Is there any SaaS offering that lets you quickly create a provisioned, usage based API for your existing cloud services?

Let me explain.

Let's say I have some code that transforms widgets. Right now, I'm the only person who could invoke it.

I would ideally like to sell access to it. I would integrate with some SaaS API-as-a-service. They would handle creating user accounts, multi-user access, 2FA, billing, and monitoring usage.

I would set them up with an endpoint to my service along with the expected inputs. I would be responsible for hosting the API endpoint as well as supplying the containers to execute the services.

Perhaps they would request modifying the function to include an additional parameter for tracking purposes.

Then they would be billed either based on usage or a flat fee..

My guess is that maybe the solution involves serverless somehow.