## Summary
Fixes: elastic#155154 (introduced in elastic#149878), builds on elastic#155436 .

- Adds tests to ensure the configuration merging order, check those for
reference.
- Updates the README to explain the intention

For the tests, I needed to output something to the logs. I hope it's not
a big issue to log it. If needed, I might hide that behind a verbose- or
feature flag.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

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Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>
(cherry picked from commit c57589e)

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