also compute completion stats for 2.x shards

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -680,6 +680,10 @@ public class IndexShard extends AbstractIndexShardComponent {
         CompletionStats completionStats = new CompletionStats();
         try (final Engine.Searcher currentSearcher = acquireSearcher("completion_stats")) {
             completionStats.add(CompletionFieldStats.completionStats(currentSearcher.reader(), fields));
+            // Necessary for 2.x shards:
+            Completion090PostingsFormat postingsFormat = ((Completion090PostingsFormat)
+                PostingsFormat.forName(Completion090PostingsFormat.CODEC_NAME));
+            completionStats.add(postingsFormat.completionStats(currentSearcher.reader(), fields));
         }
         return completionStats;
     }