API to update subscription metadata

[rishi-raj-jain]

Feature request

From a ticket and another and another

Merchants store internal identifiers in subscription.metadata to grant access to the correct in-app account. When a customer loses access to their sign-in email, they may need to transfer the access to a new account but keep billing unchanged. There is currently no API to update subscription metadata (not customer metadata), creating challenges for them. An API endpoint for updating subscription metadata would solve this problem.