Back to HashSet in metrics collector

It was not the cause of the problem.

Author: acogoluegnes

src/main/java/com/rabbitmq/client/impl/AbstractMetricsCollector.java

@@ -342,7 +342,7 @@ private static class ChannelState {
 
         final Set<Long> unackedMessageDeliveryTags = new HashSet<>();
         final Set<String> consumersWithManualAck = new HashSet<>();
-        final Set<Long> unconfirmedMessageDeliveryTags = ConcurrentHashMap.newKeySet();
+        final Set<Long> unconfirmedMessageDeliveryTags = new HashSet<>();
 
         final Channel channel;