title: Parallel processing and auto scaling in spring-kafka KafkaListener
body: <p>I'm using spring-kafka to consume messages from two Kafka topics, which sends same message format as below. </p>
  
  <pre><code>    @KafkaListener(topics = {"topic_country1", "topic_country2"}, groupId = KafkaUtils.MESSAGE_GROUP)
      public void onCustomerMessage(String message, Acknowledgment ack) throws Exception {
          log.info("Message : {}  is received", message);
          ack.acknowledge();
      }
  </code></pre>
  
  <ul>
  <li>Can KafkaListener allocate the number of consumer threads according to the number of topics that it listens by it's own and parallel process messages in two topics? Or it doesn't support parallel processing and messages have to wait in the topic till one message gets processed?</li>
  <li>In case if the number of messages in the topic is higher, I need to autoscale my micro-service to start new instances (till the number of partitions). What are the parameters (CPU, memory) I can depend on to find out the number of messages in the topics is higher from KafkaListener point of view? (i.e In an API I can auto-scale the service by monitoring the HTTP latency)</li>
  </ul>