'Get the topic subscription for the group.
For the leader, this will include the union of all member subscriptions.
For followers, it is the member\'s subscription only.
This is used when querying topic metadata to detect metadata changes
that would require rebalancing (the leader fetches metadata for all
topics in the group so that it can do partition assignment).
Returns:
set: topics'

def group_subscription(self):

    return self._group_subscription
