MINOR: Collect metadata log dir in kraft system tests (apache#12215)

It is useful to collect the directory for `__cluster_metadata` in system tests. We use a separate directory from user partitions, so it must be configured separately. 

Reviewers: David Arthur <[email protected]>

tests/kafkatest/services/kafka/kafka.py

@@ -183,6 +183,9 @@ class for details.
         "kafka_data_2": {
             "path": DATA_LOG_DIR_2,
             "collect_default": False},
+        "kafka_cluster_metadata": {
+            "path": METADATA_LOG_DIR,
+            "collect_default": False},
         "kafka_heap_dump_file": {
             "path": HEAP_DUMP_FILE,
             "collect_default": True}