[exporter/kafka] Add factory options for custom metric and log marshalers

[bgranetzke]

Description:
Adding ExportFactory options to add custom log/metric marshalers (similar to existing: WithTracesMarshalers)

Link to tracking Issue: #14514

Testing:
Tests were added for the new options as well as adapting the previous test for WithTracesMarshalers to the new method.

[bgranetzke]

@MovieStoreGuy Sorry for creating a new PR. This should have all your recommendations incorporated. It also leverages your latest factory_test approach.

[bgranetzke]

@MovieStoreGuy It was a bit more involved than initially thought. I've removed the log-based assertion of the expected marshaler being selected and replaced with an approach that validates the marshaler by assessing the exporter output.
I'd love to get your feedback. Callouts:

1. re-introduced the ProducerFactoryFunc from the original PR. I believe the existing sarama.SyncProducer interface (along with the equivalent sarama/mocks.SyncProducer) is sufficient for testing without introducing a new abstraction.
2. addressed your original defaulting concern (vs. defaulting to nil)
3. Adapted your test cases to include producerFunc in the test scenario structs

[MovieStoreGuy]

Let me have a further look to see how i can help :)

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[MovieStoreGuy]

Once you get a chance @bgranetzke , please rebase :)

[bgranetzke]

@MovieStoreGuy Thank you for the changes. Correct me if I've missed something, but there is no longer a reason for this PR, correct? Just checking before I close it. Thanks again!

[MovieStoreGuy]

I don't believe so @bgranetzke , you should be safe to close this out.