Question from a rideshare company

A product manager on the Growth Team has proposed a new feature. Instead of getting a free ride for every successful invite, users will get 1 Surge protector, which exempts them from Surge pricing on their next surged trip.

What would you choose as the key measure of the success of the feature?

What other metrics would be worth watching in addition to the key indicator?

Describe an experiment design that you could use to confirm the hypothesis that your chosen key measure is different in the treated group.