Expose `Kafka.Consumer.subscribe`

[mniip]

This allows changing the subscribed topics mid-execution.

It looks like the function already has haddocks, so perhaps it was unexported by accident?

[AlexeyRaga]

Hmmm... Not really by accident...
In order to create KafkaConsumer you now need to pass Subscription. and newConsumer subscribes.
So, currently there is no way to create "unsubscribed" consumer.

And I don't know what happens (and if it is even legal) to take an already subscribed and running consumer, and subscribe it again... :-/

[mniip]

I can't find a piece of documentation that confirms or denies whether it's legal, but I do find some references in the librdkafka issue tracker where people are doing it, and the maintainers are treating it as a seemingly supported use case. E.g:

confluentinc/librdkafka#768 (comment)

A subscription set is atomic, that is, it replaces any previous subscription set.
The process of updating the subscription is quite heavy (offset commits + rejoin group + new rebalancing + offset fetch + start consuming), so it should be avoided if possible.