Add redelivery rate to metrics

[fkollmann]

During operations of LavinMQ, the re-delivery rate is an important indicator, if sth is wrong with the overall client application.

Currently the metrics do not show the re-delivery rate (or count?).

An alternative could be to check if the minimum lavinmq_queue_messages never reaches 0 within 1 hour, but this could easily be true in larger applications, anyway.

Please add it to the /metrics endpoint, which provides the Prometheus export.

For reference, see the RabbitMQ metric: rabbitmq_global_messages_redelivered_total

Best Regards, Felix