Align to interface tweak.

Author: dario-weswit

src/main/java/com/lightstreamer/examples/messenger_demo/adapters/IMMetadataAdapter.java

@@ -20,7 +20,6 @@
 import java.io.File;
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
-import java.util.concurrent.CompletionStage;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -94,7 +93,7 @@ public String[] getItems(String user, String session, String id, String dataAdap
      * Triggered by a client "sendMessage" call.
      * The message encodes an instant message from the client.
      */
-    public CompletionStage<String> notifyUserMessage(String user, String session, String message)
+    public CompletableFuture<String> notifyUserMessage(String user, String session, String message)
         throws NotificationException, CreditsException {
 
         // we won't introduce blocking operations, hence we can proceed inline