LavinMQ::MQTT::SubscriptionKey

Identifies an MQTT subscription within an exchange's @bindings.

Mirrors the interface of LavinMQ::AMQP::BindingKey (routing_key, arguments, properties_key) so that MQTT::SubscriptionDetails and AMQP::BindingDetails stay interchangeable through duck typing, while keeping MQTT decoupled from the AMQP binding key type.

The subscription topic filter is carried in topic_filter (also exposed as routing_key for interface parity); the QoS is carried in qos. arguments exposes the QoS as the AMQP::Table the rest of the system expects, see MQTT.qos_arguments. It is nilable only for parity with AMQP::BindingKey#arguments; it never is.