added feature flag

Author: martijnvg

server/src/main/java/org/elasticsearch/index/codec/PerFieldFormatSupplier.java

@@ -15,6 +15,7 @@
 import org.apache.lucene.codecs.lucene90.Lucene90DocValuesFormat;
 import org.apache.lucene.codecs.lucene99.Lucene99HnswVectorsFormat;
 import org.elasticsearch.common.util.BigArrays;
+import org.elasticsearch.common.util.FeatureFlag;
 import org.elasticsearch.index.IndexMode;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexVersions;
@@ -34,6 +35,8 @@
  */
 public class PerFieldFormatSupplier {
 
+    private static final FeatureFlag SEQNO_FIELD_USE_TSDB_DOC_VALUES_FORMAT = new FeatureFlag("seqno_field_use_tsdb_doc_values_format");
+
     private static final DocValuesFormat docValuesFormat = new Lucene90DocValuesFormat();
     private static final KnnVectorsFormat knnVectorsFormat = new Lucene99HnswVectorsFormat();
     private static final ES819TSDBDocValuesFormat tsdbDocValuesFormat = new ES819TSDBDocValuesFormat();
@@ -129,7 +132,7 @@ private boolean excludeFields(String fieldName) {
         return fieldName.startsWith("_")
             && fieldName.equals("_tsid") == false
             && fieldName.equals("_ts_routing_hash") == false
-            && fieldName.equals(SeqNoFieldMapper.NAME) == false;
+            && (SEQNO_FIELD_USE_TSDB_DOC_VALUES_FORMAT.isEnabled() && fieldName.equals(SeqNoFieldMapper.NAME) == false);
     }
 
     private boolean isTimeSeriesModeIndex() {