Remove ImmutableOpenMap from Engine

SegmentStats was changed to use Map, but the method in Engine which
computes files sizes for segment stats was never converted. This commit
removes that final usage from Engine.

relates elastic#86239

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -28,7 +28,6 @@
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.action.index.IndexRequest;
 import org.elasticsearch.common.bytes.BytesReference;
-import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
@@ -66,6 +65,7 @@
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -843,9 +843,9 @@ protected void fillSegmentStats(SegmentReader segmentReader, boolean includeSegm
         }
     }
 
-    private ImmutableOpenMap<String, SegmentsStats.FileStats> getSegmentFileSizes(SegmentReader segmentReader) {
+    private Map<String, SegmentsStats.FileStats> getSegmentFileSizes(SegmentReader segmentReader) {
         try {
-            final ImmutableOpenMap.Builder<String, SegmentsStats.FileStats> files = ImmutableOpenMap.builder();
+            Map<String, SegmentsStats.FileStats> files = new HashMap<>();
             final SegmentCommitInfo segmentCommitInfo = segmentReader.getSegmentInfo();
             for (String fileName : segmentCommitInfo.files()) {
                 String fileExtension = IndexFileNames.getExtension(fileName);
@@ -857,11 +857,11 @@ private ImmutableOpenMap<String, SegmentsStats.FileStats> getSegmentFileSizes(Se
                         logger.warn(() -> "Error when retrieving file length for [" + fileName + "]", ioe);
                     } catch (AlreadyClosedException ace) {
                         logger.warn(() -> "Error when retrieving file length for [" + fileName + "], directory is closed", ace);
-                        return ImmutableOpenMap.of();
+                        return Map.of();
                     }
                 }
             }
-            return files.build();
+            return Collections.unmodifiableMap(files);
         } catch (IOException e) {
             logger.warn(
                 () -> format(
@@ -871,7 +871,7 @@ private ImmutableOpenMap<String, SegmentsStats.FileStats> getSegmentFileSizes(Se
                 ),
                 e
             );
-            return ImmutableOpenMap.of();
+            return Map.of();
         }
     }