Tiago Perini

Certain publisher metrics are no longer available for MQTT publishers after 3.12.0

2

### Describe the bug After upgrading from RabbitMQ 3.11 to 3.12, we noticed that metrics such as `rabbitmq_queue_messages_published_total`, `rabbitmq_channel_messages_delivered_total`, `rabbitmq_channel_messages_delivered_ack_total` and `rabbitmq_channel_messages_redelivered_total` are no longer available, but only for queues/messages...