KafkaAdminClient no longer retrieving all partitions by default

[etripier]

#2573 introduced a regression for the KafkaAdminClient - previously, you would be able to instantiate it and call admin_client.cluster.available_partitions_for_topic(topic_name) to automatically retrieve partitions for the given topic, but now the set of partitions is always empty because the metadata request is for NO_TOPICS.

Do you have a recommendation for achieving the old behavior? Maybe KafkaAdminClient could just always fetch metadata for all topics? The change looks like it was intended to fix the case of a bad producer having too much access, but that's not really a concern for the admin client.

[dpkp]

Try admin_client.describe_topics([topic_name])