add comment about apache/pulsar#11553

Author: eolivelli

kafka-impl/src/main/java/io/streamnative/pulsar/handlers/kop/coordinator/transaction/PulsarStorageProducerIdManagerImpl.java

@@ -108,6 +108,8 @@ private synchronized void endReadLoop(CompletableFuture<?> handle) {
 
     @Override
     public synchronized CompletableFuture<Long> generateProducerId() {
+        // we could get rid of the Exclusive Producer if we had Message.getIndex()
+        // introduced in 2.9.0 https://github.com/apache/pulsar/pull/11553
         CompletableFuture<Producer<byte[]>> producerHandle = pulsarClient.newProducer()
                 .enableBatching(false)
                 .topic(topic)