test: added extra log line

Relates to #25311

qa/full-cluster-restart/src/test/java/org/elasticsearch/upgrades/FullClusterRestartIT.java

@@ -478,6 +478,7 @@ public class FullClusterRestartIT extends ESRestTestCase {
             // Post upgrade checks:
             assertBusy(() -> {
                 Map<String, Object> rsp2 = toMap(client().performRequest("GET", "/" + index + "/_upgrade"));
+                logger.info("upgrade status response: {}", rsp2);
                 Map<?, ?> indexUpgradeStatus2 = (Map<?, ?>) XContentMapValues.extractValue("indices." + index, rsp2);
                 int totalBytes2 = (Integer) indexUpgradeStatus2.get("size_in_bytes");
                 assertThat(totalBytes2, greaterThan(0));