Experiment with using tsdb doc values codec for _seq_no. (#133788)

Running metricsgenreceiver without this change:

```
"_seq_no": {
    "total": "369.9mb",
    "total_in_bytes": 387962443,
    "inverted_index": {
        "total": "0b",
        "total_in_bytes": 0
    },
    "stored_fields": "0b",
    "stored_fields_in_bytes": 0,
    "doc_values": "369.9mb",
    "doc_values_in_bytes": 387962443,
    "points": "0b",
    "points_in_bytes": 0,
    "norms": "0b",
    "norms_in_bytes": 0,
    "term_vectors": "0b",
    "term_vectors_in_bytes": 0,
    "knn_vectors": "0b",
    "knn_vectors_in_bytes": 0
}
```

and running metricsgenreceiver with this change:

```
"_seq_no": {
    "total": "212.3mb",
    "total_in_bytes": 222616293,
    "inverted_index": {
        "total": "0b",
        "total_in_bytes": 0
    },
    "stored_fields": "0b",
    "stored_fields_in_bytes": 0,
    "doc_values": "212.3mb",
    "doc_values_in_bytes": 222616293,
    "points": "0b",
    "points_in_bytes": 0,
    "norms": "0b",
    "norms_in_bytes": 0,
    "term_vectors": "0b",
    "term_vectors_in_bytes": 0,
    "knn_vectors": "0b",
    "knn_vectors_in_bytes": 0
}
```

metricsgenreceiver config:

```
receivers:
  metricsgen:
    start_now_minus: 72h
    interval: 5s
    interval_jitter_std_dev: 1ms
    real_time: false
    exit_after_end: true
    seed: 123
    scenarios:
      - path: builtin/hostmetrics
        scale: 100
```

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;
@@ -25,6 +26,7 @@
 import org.elasticsearch.index.mapper.IdFieldMapper;
 import org.elasticsearch.index.mapper.Mapper;
 import org.elasticsearch.index.mapper.MapperService;
+import org.elasticsearch.index.mapper.SeqNoFieldMapper;
 import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper;
 
 /**
@@ -33,6 +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();
@@ -122,9 +126,13 @@ boolean useTSDBDocValuesFormat(final String field) {
     }
 
     private boolean excludeFields(String fieldName) {
+        // TODO: should we just allow all fields to use tsdb doc values codec?
         // Avoid using tsdb codec for fields like _seq_no, _primary_term.
         // But _tsid and _ts_routing_hash should always use the tsdb codec.
-        return fieldName.startsWith("_") && fieldName.equals("_tsid") == false && fieldName.equals("_ts_routing_hash") == false;
+        return fieldName.startsWith("_")
+            && fieldName.equals("_tsid") == false
+            && fieldName.equals("_ts_routing_hash") == false
+            && (SEQNO_FIELD_USE_TSDB_DOC_VALUES_FORMAT.isEnabled() && fieldName.equals(SeqNoFieldMapper.NAME) == false);
     }
 
     private boolean isTimeSeriesModeIndex() {