[DISCUSS] PIP-253: Expose producer metrics for deadLetterProducer and retryLetterProducer

[Kai Levy]

Hello!

I created a new PIP because I discovered there's no way for a user to
access the metrics for a consumer's deadLetterProducer /
retryLetterProducer, since it is private to ConsumerImpl.java. I would like
to propose an API change that would expose those statistics. More details
on the github issue:
https://github.com/apache/pulsar/issues/19698

Thanks!
Kai