fix(spanner): Add logs

Author: sakthivelmanii

google-cloud-spanner/src/main/java/com/google/cloud/spanner/AsyncResultSetImpl.java

@@ -483,6 +483,7 @@ public void run() {
   @Override
   public ApiFuture<Void> setCallback(Executor exec, ReadyCallback cb) {
     synchronized (monitor) {
+      System.out.println("setCallback is called");
       Preconditions.checkState(!closed, "This AsyncResultSet has been closed");
       Preconditions.checkState(
           this.state == State.INITIALIZED, "callback may not be set multiple times");