@type t() :: %PolarExpress.Schemas.SubscriptionCreatedMetadata{
  amount: integer() | nil,
  currency: String.t() | nil,
  product_id: String.t() | nil,
  recurring_interval: String.t() | nil,
  recurring_interval_count: integer() | nil,
  started_at: String.t() | nil,
  subscription_id: String.t() | nil
}