Actually update the metric pubsub-await-update

Signed-off-by: Cai Yufei (INST/ECS1) <yufei.cai@bosch-si.com>

services/utils/pubsub/src/main/java/org/eclipse/ditto/services/utils/pubsub/actors/SubUpdater.java

@@ -285,6 +285,7 @@ private void enqueueRequest(final Request request, final boolean changed) {
         if (request.shouldAcknowledge()) {
             final Acknowledgement acknowledgement = Acknowledgement.of(request, getSender());
             awaitUpdate.add(acknowledgement);
+            awaitUpdateMetric.increment();
         }
     }