Round-robin multiple Kafka topics

[lyradc]

Allows the topic parameter to contain multiple topics delimited by comma (,).

This is to satisfy request #281 .

Initial testing shows fairly consistent distribution of logs across topics.

[vruello]

Hi @lyradc,

I'm a bit concerned about the performance impact of using an atomic. That's why I proposed to use a simple "thread-local" counter in the write method. Since clients usually send much more events than there are Kafka topics to write to, this should provide a reasonable distribution across topics.

In practice, this would mean replacing l_topic_index with a simple uint local to the write method, initialized to 0.

What do you think?

[lyradc]

Thank you for the feedback @vruello. I was concerned at lower volume this wouldn't distribute well... However after implementing a local uint in the write fn I observed an even distribution across topics at low volume.

[vruello]

I made some changes:

• The list of topics does not need to be computed for each batch of events. I computed it once in KafkaOutput::new and stored the result in KafkaOutput.topics.
• Since KafkaConfiguration::topic_as_array is only used once by the Kafka driver, it seems cleaner to replace it with inlined code in KafkaOutput::new.
• Since KafkaOutput.config was only used to retrieve the destination topic, it can now be removed.

@lyradc are you ok with these changes?

[lyradc]

Thanks again, yeah that looks good to me.