Refresh translog stats after translog trimming in NoOpEngine (elastic#43825)

This commit changes NoOpEngine so that it refreshes its translog 
stats once translog is trimmed.

Relates elastic#43156

Author: tlrx

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

@@ -158,6 +158,8 @@ public void trimUnreferencedTranslogFiles() {
                     try (Translog translog = new Translog(translogConfig, translogUuid, translogDeletionPolicy,
                         engineConfig.getGlobalCheckpointSupplier(), engineConfig.getPrimaryTermSupplier(), seqNo -> {})) {
                         translog.trimUnreferencedReaders();
+                        // refresh the translog stats
+                        this.translogStats = translog.stats();
                     }
                 }
             }

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

@@ -75,13 +75,14 @@ public class ReadOnlyEngine extends Engine {
         BlockTreeTermsReader.FSTLoadMode.AUTO.name());
     private final SegmentInfos lastCommittedSegmentInfos;
     private final SeqNoStats seqNoStats;
-    private final TranslogStats translogStats;
     private final SearcherManager searcherManager;
     private final IndexCommit indexCommit;
     private final Lock indexWriterLock;
     private final DocsStats docsStats;
     private final RamAccountingSearcherFactory searcherFactory;
 
+    protected volatile TranslogStats translogStats;
+
     /**
      * Creates a new ReadOnlyEngine. This ctor can also be used to open a read-only engine on top of an already opened
      * read-write engine. It allows to optionally obtain the writer locks for the shard which would time-out if another

server/src/test/java/org/elasticsearch/index/engine/NoOpEngineTests.java

@@ -195,8 +195,15 @@ public void testTrimUnreferencedTranslogFiles() throws Exception {
         }
 
         assertThat(Translog.readMinTranslogGeneration(translogPath, translogUuid), equalTo(minFileGeneration));
+        assertThat(noOpEngine.getTranslogStats().estimatedNumberOfOperations(), equalTo(numDocs));
+        assertThat(noOpEngine.getTranslogStats().getUncommittedOperations(), equalTo(0));
+
         noOpEngine.trimUnreferencedTranslogFiles();
+
         assertThat(Translog.readMinTranslogGeneration(translogPath, translogUuid), equalTo(lastCommitedTranslogGeneration));
+        assertThat(noOpEngine.getTranslogStats().estimatedNumberOfOperations(), equalTo(0));
+        assertThat(noOpEngine.getTranslogStats().getUncommittedOperations(), equalTo(0));
+
         noOpEngine.close();
     }