How would you measure the success of stripe billing?

I would first confirm that Stripe Billing is the feature that allows a merchant to invoice their end-customers or enable them to sign up for subscriptions. This product is available in 3 flavors:
1. Basic

2. Professional

3. Enterprise

Each catering to a different customer-segment.

 Would ask additional clarification question on if there is a specific aspect that the interviewer wants me to focus on as the success of a product can vary depending on whether we define it based on:

1. Adoption

2. Continued Usage

3. Revenue  and or profitability.

Assuming the interviewer wants me to pick one or more critical success metrics for each of these:

1. Adoption:

a. Month over Month (MoM) growth in the number of customers signing up to use Stripe Billing.

2. Continued usage:

a. DAU or MAU of any critical features or pages in the product.

b. MoM churn rate. We want to keep the churn rate as low as possible as it will boost the average Lifetime Value (LTV) of a customer

3. Revenue

Stripe's typical revenue model is based on a percentage of overall transactions processed. Hence it would be important to looks at:

a. Average MoM transactions processed in terms of $.

b. Average MoM revenue per customer.