[receiver/kafka] max_fetch_size consumer option is not used in franz-go

[paulojmdias]

Component(s)

No response

Describe the issue you're reporting

Description

Based on the code below, we are using the default_fetch_size config value in the kgo.FetchMaxBytes option instead of max_fetch_size, and that can lead to confusion in the user's configuration, as we don't have any reference to it in the README.md.

opentelemetry-collector-contrib/internal/kafka/franz_client.go

Lines 132 to 134 in 1d59fde

	if consumerCfg.DefaultFetchSize > 0 {
	opts = append(opts, kgo.FetchMaxBytes(consumerCfg.DefaultFetchSize))
	}

I'm opening this ticket to discuss whether we should only update the README.md to reference this behaviour, update the code above to use the max_fetch_size instead of default_fetch_size, and plan to remove default_fetch_size while we remove Sarama completely.

Whatever we decide to do, if we decide to, I'm open to open a PR for it 👍

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