setAttributes method
Attaches attributes to the current subscriber so the billing vendor can
segment revenue by them.
With RevenueCat this maps to subscriber attributes, which then appear in
charts and exports — that's what turns a captured code into a payable
affiliate figure. One adapter class commonly implements both this and
EntitlementService, since the same SDK backs them.
Pass a null value to clear a key.
Implementation
@override
Future<void> setAttributes(Map<String, String?> next) =>
Future.sync(() => attributes.addAll(next));