Fix LogsdbIndexingRollingUpgradeIT (#133562)

The template that this test suite creates was never used during the test. This is because it uses default priority, and the builtin `logs@template` which also matches with the data stream name has a higher priority.

This change addresses this problem by setting the priority to 200 for the template that this test suite creates. Additionally, an extra check is added the verifying that the created template is actually used by the data stream that this test suite creates.

This change results in this test suite to fail if #133493 isn't applied.

Author: martijnvg

x-pack/plugin/logsdb/qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/upgrades/LogsdbIndexingRollingUpgradeIT.java

@@ -34,6 +34,7 @@
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
+import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.notNullValue;
 
 public class LogsdbIndexingRollingUpgradeIT extends AbstractRollingUpgradeWithSecurityTestCase {
@@ -78,7 +79,8 @@ public void testIndexing() throws Exception {
         if (isOldCluster()) {
             startTrial();
             enableLogsdbByDefault();
-            createTemplate(dataStreamName, getClass().getSimpleName().toLowerCase(Locale.ROOT), TEMPLATE);
+            String templateId = getClass().getSimpleName().toLowerCase(Locale.ROOT);
+            createTemplate(dataStreamName, templateId, TEMPLATE);
 
             Instant startTime = Instant.now().minusSeconds(60 * 60);
             bulkIndex(dataStreamName, 4, 1024, startTime);
@@ -88,6 +90,8 @@ public void testIndexing() throws Exception {
             assertThat(((Map<?, ?>) settings.get("settings")).get("index.mode"), equalTo("logsdb"));
             assertThat(((Map<?, ?>) settings.get("defaults")).get("index.mapping.source.mode"), equalTo("SYNTHETIC"));
 
+            // check prior to rollover
+            assertDataStream(dataStreamName, templateId);
             ensureGreen(dataStreamName);
             search(dataStreamName);
             query(dataStreamName);
@@ -115,9 +119,20 @@ public void testIndexing() throws Exception {
         }
     }
 
+    static void assertDataStream(String dataStreamName, String templateId) throws IOException {
+        var getDataStreamsRequest = new Request("GET", "/_data_stream/" + dataStreamName);
+        var getDataStreamResponse = client().performRequest(getDataStreamsRequest);
+        assertOK(getDataStreamResponse);
+        var dataStreams = entityAsMap(getDataStreamResponse);
+        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.name"), equalTo(dataStreamName));
+        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.indices"), hasSize(1));
+        assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.template"), equalTo(templateId));
+    }
+
     static void createTemplate(String dataStreamName, String id, String template) throws IOException {
         final String INDEX_TEMPLATE = """
             {
+                "priority": 200,
                 "index_patterns": ["$DATASTREAM"],
                 "template": $TEMPLATE,
                 "data_stream": {