Checkout API: Support custom metadata

[birkjernstrom]

Allowing developers to store metadata (JSONB) associated with a checkout request, e.g user_id of the applications user to map to a subscription. Accessible post checkout and in webhooks for orders to support secure custom logic and implementations.

[de-mawo]

Where can I find this implementation on the Docs?. I have a nextjs app. I would like my Checkout page to have the email and name prefilled with details of my App User as shown here #4041 . I also want my App user_Id to be added to the metadata