ClickHouse/tests/integration/test_cluster_discovery/config/config_observer.xml

<clickhouse>
    <allow_experimental_cluster_discovery/>
    <remote_servers>
        <test_auto_cluster>
            <discovery>
                <observer/>
            </discovery>
        </test_auto_cluster>
        <two_shards>
            <!-- just to check that there's no conflict between automatic and manual clusters -->
            <shard>
                <replica>
                    <host>node1</host>
                    <port>9000</port>
                </replica>
                <replica>
                    <host>node2</host>
                    <port>9000</port>
                </replica>
            </shard>
        </two_shards>
    </remote_servers>
</clickhouse>