Fix a race condition in WatcherCleanerTest.testDeadWatcherMetrics

Because the metrics were updated _after_ the listener is invoked, the listener does not always see
the fresh metric value. This fixes it so that the updated metric value is always visible to the
listener.

zookeeper-server/src/main/java/org/apache/zookeeper/server/watch/WatcherCleaner.java

@@ -158,13 +158,13 @@ public void run() {
                 cleaners.schedule(new WorkRequest() {
                     @Override
                     public void doWork() throws Exception {
+                        ServerMetrics.getMetrics().DEAD_WATCHERS_CLEARED.add(total);
+                        totalDeadWatchers.addAndGet(-total);
                         long startTime = Time.currentElapsedTime();
                         listener.processDeadWatchers(snapshot);
                         long latency = Time.currentElapsedTime() - startTime;
                         LOG.info("Takes {} to process {} watches", latency, total);
                         ServerMetrics.getMetrics().DEAD_WATCHERS_CLEANER_LATENCY.add(latency);
-                        ServerMetrics.getMetrics().DEAD_WATCHERS_CLEARED.add(total);
-                        totalDeadWatchers.addAndGet(-total);
                         synchronized (processingCompletedEvent) {
                             processingCompletedEvent.notifyAll();
                         }