Add missing index.mapping.total_fields.limit setting to the target index (#89875)

When the source index has a non-default value
for the total number of fields in the mapping we
need to copy that value and apply the same setting
to the target index.

x-pack/plugin/rollup/src/main/java/org/elasticsearch/xpack/downsample/TransportRollupAction.java

@@ -558,6 +558,10 @@ public class TransportRollupAction extends AcknowledgedTransportMasterNodeAction
                 .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
                 .put(IndexSettings.INDEX_REFRESH_INTERVAL_SETTING.getKey(), "-1")
                 .put(IndexMetadata.INDEX_DOWNSAMPLE_STATUS.getKey(), IndexMetadata.DownsampleTaskStatus.STARTED)
+                .put(
+                    MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey(),
+                    sourceIndexMetadata.getSettings().get(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())
+                )
                 .build()
         ).mappings(mapping);
         clusterService.submitStateUpdateTask("create-rollup-index [" + rollupIndexName + "]", new RollupClusterStateUpdateTask(listener) {

x-pack/plugin/rollup/src/test/java/org/elasticsearch/xpack/downsample/DownsampleActionSingleNodeTests.java

@@ -42,6 +42,7 @@ import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.engine.VersionConflictEngineException;
 import org.elasticsearch.index.mapper.DateFieldMapper;
+import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.TimeSeriesIdFieldMapper;
 import org.elasticsearch.index.mapper.TimeSeriesParams;
 import org.elasticsearch.indices.IndicesService;
@@ -283,11 +284,16 @@ public class DownsampleActionSingleNodeTests extends ESSingleNodeTestCase {
     }
 
     public void testCopyIndexSettings() throws IOException {
-        Settings settings = Settings.builder()
+        final Settings.Builder settingsBuilder = Settings.builder()
             .put(LifecycleSettings.LIFECYCLE_NAME, randomAlphaOfLength(5))
             .put(RolloverAction.LIFECYCLE_ROLLOVER_ALIAS_SETTING.getKey(), randomAlphaOfLength(5))
-            .put(LifecycleSettings.LIFECYCLE_PARSE_ORIGINATION_DATE_SETTING.getKey(), randomBoolean())
-            .build();
+            .put(LifecycleSettings.LIFECYCLE_PARSE_ORIGINATION_DATE_SETTING.getKey(), randomBoolean());
+
+        final Integer totalFieldsLimit = randomBoolean() ? randomIntBetween(100, 10_000) : null;
+        if (totalFieldsLimit != null) {
+            settingsBuilder.put(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey(), totalFieldsLimit);
+        }
+        final Settings settings = settingsBuilder.build();
         logger.info("Updating index [{}] with settings [{}]", sourceIndex, settings);
 
         var updateSettingsReq = new UpdateSettingsRequest(settings, sourceIndex);
@@ -816,6 +822,16 @@ public class DownsampleActionSingleNodeTests extends ESSingleNodeTestCase {
             indexSettingsResp.getSetting(sourceIndex, IndexMetadata.SETTING_INDEX_HIDDEN),
             indexSettingsResp.getSetting(rollupIndex, IndexMetadata.SETTING_INDEX_HIDDEN)
         );
+
+        if (indexSettingsResp.getSetting(sourceIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()) != null) {
+            assertNotNull(indexSettingsResp.getSetting(rollupIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()));
+        }
+        if (indexSettingsResp.getSetting(sourceIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()) != null
+            && indexSettingsResp.getSetting(rollupIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()) != null)
+            assertEquals(
+                indexSettingsResp.getSetting(sourceIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey()),
+                indexSettingsResp.getSetting(rollupIndex, MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())
+            );
     }
 
     private AggregationBuilder buildAggregations(