While both indicate a mismatch between the broker and client, we want to be able to differentiate between whether the error is thrown by the client vs the broker.
This was completely broken previously because it didn't lookup the group coordinator of the consumer group. Also added basic error handling/raising. Note: I added the `group_coordinator_id` as an optional kwarg. As best I can tell, the Java client doesn't include this and instead looks it up every time. However, if we add this, it allows the caller the flexibility to bypass the network round trip of the lookup if for some reason they already know the `group_coordinator_id`.
Set a clear default value for `validate_only` / `include_synonyms` Previously the kwarg defaulted to `None`, but then sent a `False` so this makes it more explicit and reduces ambiguity.
Previously, this only queried the controller. In actuality, the Kafka protocol requires that the client query all brokers in order to get the full list of consumer groups. Note: The Java code (as best I can tell) doesn't allow limiting this to specific brokers. And on the surface, this makes sense... you typically don't care about specific brokers. However, the inverse is true... consumer groups care about knowing their group coordinator so they don't have to repeatedly query to find it. In fact, a Kafka broker will only return the groups that it's a coordinator for. While this is an implementation detail that is not guaranteed by the upstream broker code, and technically should not be relied upon, I think it very unlikely to change. So monitoring scripts that fetch the offsets or describe the consumers groups of all groups in the cluster can simply issue one call per broker to identify all the coordinators, rather than having to issue one call per consumer group. For an ad-hoc script this doesn't matter, but for a monitoring script that runs every couple of minutes, this can be a big deal. I know in the situations where I will use this, this matters more to me than the risk of the interface unexpectedly breaking.
Support fetching the offsets of a consumer group. Note: As far as I can tell (the Java code is a little inscrutable), the Java AdminClient doesn't allow specifying the `coordinator_id` or the `partitions`. But I decided to include them because they provide a lot of additional flexibility: 1. allowing users to specify the partitions allows this method to be used even for older brokers that don't support the OffsetFetchRequest_v2 2. allowing users to specify the coordinator ID gives them a way to bypass a network round trip. This method will frequently be used for monitoring, and if you've got 1,000 consumer groups that are being monitored once a minute, that's ~1.5M requests a day that are unnecessarily duplicated as the coordinator doesn't change unless there's an error.