No nng_mqtt_msg_free API provided

[wanghaEMQ]

Is your feature request related to a problem? Please describe.
I create a MQTT msg by calling nng_mqtt_msg_alloc. When I want to free it. No corresponding API was found. Should I call nng_msg_free?

Describe the solution you'd like
Please provide an API nng_mqtt_msg_free. Thanks

Describe alternatives you've considered
.

Additional context
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