Metadata is a key/value object that can be attached to Payments, Invoices, and Tokens.
You can specify up to 30 keys, with key names up to 40 characters long and values up to 500 characters long.
Metadata is useful for storing additional, structured information on an object. For example, you could store your user’s full name, email, and internal order id.
Do not store any sensitive information (bank account numbers, card details, etc.) as metadata.
Individual keys can be unset by posting an empty value to them. All keys can be unset by posting an empty value to metadata.