ducktape: run GroupMetricsTest with group_topic_partitions=1

test_leadership_transfer relies on kafka_group_offset metric source as an
indication of the node being a group coordinator. However in admin API
there is no way to know which __consumer_groups partition a specific
consumer group uses, and the testcase always works with partition 0.
To make that work, number of __consumer_groups partition is set to 1
explicitly.

tests/rptest/tests/group_membership_test.py

@@ -127,7 +127,8 @@ def __init__(self, ctx, *args, **kwargs):
 
         # Require internal_kafka topic to have an increased replication factor
         extra_rp_conf = dict(default_topic_replications=3,
-                             enable_leader_balancer=False)
+                             enable_leader_balancer=False,
+                             group_topic_partitions=1)
         super(GroupMetricsTest, self).__init__(test_context=ctx,
                                                num_brokers=3,
                                                extra_rp_conf=extra_rp_conf)
@@ -374,6 +375,7 @@ def select_next_leader():
                        timeout_sec=30,
                        backoff_sec=5)
 
+            self.logger.debug(f"Waiting for metrics from the single node: {new_leader.account.hostname}")
             wait_until(lambda: metrics_from_single_node(new_leader),
                        timeout_sec=30,
                        backoff_sec=5)