Add log when download completes with its size

Signed-off-by: Gaurav Bafna <gbbafna@amazon.com>

server/src/main/java/org/opensearch/index/store/RemoteStoreFileDownloader.java

@@ -149,6 +149,7 @@ private void copyOneFile(
                 try {
                     cancellableThreads.executeIO(() -> {
                         destination.copyFrom(source, file, file, IOContext.DEFAULT);
+                        logger.trace("Downloaded file {} of size {}", file, destination.fileLength(file));
                         onFileCompletion.run();
                         if (secondDestination != null) {
                             secondDestination.copyFrom(destination, file, file, IOContext.DEFAULT);