bulk api: use `System.nanoTime()` instead of `System.currentTimeMillis()` to compute the estimated took time

In this case we compute elapsed time and `System.nanoTime()` is designed to do just do that.
The absolute timing `System.currentTimeMillis()` provides in not needed and the relative timing `System.nanoTime()` provides is likely to be more accurate.

core/src/main/java/org/elasticsearch/action/bulk/TransportBulkAction.java

@@ -61,6 +61,7 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 
 /**
@@ -195,16 +196,15 @@ public class TransportBulkAction extends HandledTransportAction<BulkRequest, Bul
      * @see #doExecute(BulkRequest, org.elasticsearch.action.ActionListener)
      */
     public void executeBulk(final BulkRequest bulkRequest, final ActionListener<BulkResponse> listener) {
-        final long startTime = System.currentTimeMillis();
-        executeBulk(bulkRequest, startTime, listener, new AtomicArray<>(bulkRequest.requests.size()));
+        final long startTimeNanos = System.nanoTime();
+        executeBulk(bulkRequest, startTimeNanos, listener, new AtomicArray<>(bulkRequest.requests.size()));
     }
 
-    private long buildTookInMillis(long startTime) {
-        // protect ourselves against time going backwards
-        return Math.max(1, System.currentTimeMillis() - startTime);
+    private long buildTookInMillis(long startTimeNanos) {
+        return TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTimeNanos);
     }
 
-    private void executeBulk(final BulkRequest bulkRequest, final long startTime, final ActionListener<BulkResponse> listener, final AtomicArray<BulkItemResponse> responses ) {
+    private void executeBulk(final BulkRequest bulkRequest, final long startTimeNanos, final ActionListener<BulkResponse> listener, final AtomicArray<BulkItemResponse> responses ) {
         final ClusterState clusterState = clusterService.state();
         // TODO use timeout to wait here if its blocked...
         clusterState.blocks().globalBlockedRaiseException(ClusterBlockLevel.WRITE);
@@ -302,7 +302,7 @@ public class TransportBulkAction extends HandledTransportAction<BulkRequest, Bul
         }
 
         if (requestsByShard.isEmpty()) {
-            listener.onResponse(new BulkResponse(responses.toArray(new BulkItemResponse[responses.length()]), buildTookInMillis(startTime)));
+            listener.onResponse(new BulkResponse(responses.toArray(new BulkItemResponse[responses.length()]), buildTookInMillis(startTimeNanos)));
             return;
         }
 
@@ -352,7 +352,7 @@ public class TransportBulkAction extends HandledTransportAction<BulkRequest, Bul
                 }
 
                 private void finishHim() {
-                    listener.onResponse(new BulkResponse(responses.toArray(new BulkItemResponse[responses.length()]), buildTookInMillis(startTime)));
+                    listener.onResponse(new BulkResponse(responses.toArray(new BulkItemResponse[responses.length()]), buildTookInMillis(startTimeNanos)));
                 }
             });
         }