Acquire stats searcher for data stream stats (elastic#117953) (elastic#118005)

Here, we only need to extract the minimum and maximum values of the 
timestamp field; therefore, using a stats searcher should suffice. This
is important for frozen indices.

Author: dnhatn

docs/changelog/117953.yaml

@@ -0,0 +1,5 @@
+pr: 117953
+summary: Acquire stats searcher for data stream stats
+area: Data streams
+type: bug
+issues: []

modules/data-streams/src/main/java/org/elasticsearch/datastreams/action/DataStreamsStatsTransportAction.java

@@ -31,6 +31,7 @@
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.IndexService;
 import org.elasticsearch.index.engine.Engine;
+import org.elasticsearch.index.engine.ReadOnlyEngine;
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.index.store.StoreStats;
 import org.elasticsearch.indices.IndicesService;
@@ -130,7 +131,7 @@ protected void shardOperation(
             DataStream dataStream = indexAbstraction.getParentDataStream();
             assert dataStream != null;
             long maxTimestamp = 0L;
-            try (Engine.Searcher searcher = indexShard.acquireSearcher("data_stream_stats")) {
+            try (Engine.Searcher searcher = indexShard.acquireSearcher(ReadOnlyEngine.FIELD_RANGE_SEARCH_SOURCE)) {
                 IndexReader indexReader = searcher.getIndexReader();
                 byte[] maxPackedValue = PointValues.getMaxPackedValue(indexReader, DataStream.TIMESTAMP_FIELD_NAME);
                 if (maxPackedValue != null) {