feat: add get_coordinator function in group subscriber v2

[yastanotheruser]

Add an API function to get the group subscriber process coordinator PID. This allows to dynamically change the topics the group subscribes to by calling brod_group_coordinator:update_topics.

[zmstone]

to lower the chance of race condition, maybe delete the gen_call to group subscriber v2.

[yastanotheruser]

sorry I'm a bit slow. how could I read the state of the subscriber without a call to the gen_server?

[zmstone]

sorry I'm a bit slow. how could I read the state of the subscriber without a call to the gen_server?

hm.... my bad, I must have lost a few lines when copy-paste.
The race condition (although very unlikely to happen) is:

• Application code calls get_coordinator
• The coordinator process has to restart
• Application code calls update_topics

If what you want is to be able to call brod_coordinator:update_topics, I would suggest to expose brod_group_subscriber_v2:update_topics which delegates the gen_cast to coordinator process.

And add a test case to verify that new subscribers will be started and messages will be fetched after the function is called.

[zmstone]

We can still add get_coordinator API, maybe it can be useful for some other purposes too.