[Suggestion] Send message in batch for K6 kafka output

Dear K6 support,

We found for k6 kafka output, send message in batch is much more efficient, otherwise it will take much time to send k6 raw data result to kafka.

Our code change for k6/stats/kafka/collector.go(pushMetrics() method), could u pls kindly check? thx!

Thanks,
Roy

Feel free to submit a pull request in our github repo, that’s where we can review and discuss such changes: https://github.com/loadimpact/k6/pulls