partitionsConsumedConcurrently > 1 overloads CPU when consumers lag is 0

[maksym-opanasenko-ft]

Environment Information

• Reproduced on Mac (silicone) and confirmed in AWS ECS:
• Node Version: v18.20.4
• NPM Version: 10.7.0
• Confluent Platform : 7.7.x, 7.5.x, 7.3.x; also confirmed with Apache Kafka 3.3.x
• confluent-kafka-javascript version: ^0.5.2

Steps to Reproduce

1. Launch Kafka in docker or in AWS MSK
2. Create a topic with many partitions
3. Instantiate a consumer using the kafkaJs constructor:

	const consumer = kafka.consumer({
		kafkaJS: {
			groupId: 'test-group',
		},
	});

4. Run the consumer with partitionsConsumedConcurrently > 1

	await consumer.run({
		partitionsConsumedConcurrently: 2, // This causes the CPU load
		eachMessage: async ({ topic, partition, message }) => {
			console.log({
				topic,
				partition,
				offset: message.offset,
				value: message.value.toString(),
			});
		},
	});

5. Don't write any messages to the topic or wait for the consumer to catch up with the lag
6. Wait for at least 30 seconds and observe the CPU usage

confluent-kafka-javascript Configuration Settings

Here is the full minimal snippet we use to reproduce:

const { Kafka } = require('@confluentinc/kafka-javascript').KafkaJS;

const topic = 'TestTopic';
const clientId = 'test-client-id';

const kafka = new Kafka({
	kafkaJS: {
		clientId: clientId,
		brokers: ['localhost:9092'],
		logLevel: 4, // optional, but gives a hint what overuses the CPU
	},
});

const consumerRun = async () => {
	const consumer = kafka.consumer({
		kafkaJS: {
			groupId: 'test-group',
		},
	});

	await consumer.connect();
	console.log('Consumer connected');

	await consumer.subscribe({ topic });
	console.log('Consumer subscribed to topic');

	await consumer.run({
		partitionsConsumedConcurrently: 2, // This causes the CPU load
		eachMessage: async ({ topic, partition, message }) => {
			console.log({
				topic,
				partition,
				offset: message.offset,
				value: message.value.toString(),
			});
		},
	});
};

consumerRun().catch((err) => {
	console.error('Error running producer or consumer:', err);
});

Additional context

When setting logLevel: 4 in the Kafka constructor and using partitionsConsumedConcurrently: 1, the following log message

{
  message: 'Attempting to fetch 1 messages to the message cache',
  name: 'test-client-id#consumer-1',
  fac: 'BINDING',
  timestamp: 1733298033731
}
```  is printed ~ once in a second, however, when setting `partitionsConsumedConcurrently: 2` the console is spammed with the message. 
Setting `logLevel: 3` makes it better as IO is used less, but the code that prints the message is still running causing high usage of CPU