The duplication of the instrumentation status flags raises concerns about possible duplicated flows. It also may lead to incorrect behavior in the future because the logic is maintained in multiple libraries. I would suggest to keep instrumentation logic in the core library and allow to the logback library to pass additional attributes instead.

As I mentioned earlier, I would like to prevent this flag to be manipulated externally and there is no good way to restrict access in Java to specific functionality to some packages only - please let me know if you think otherwise. I do believe maintaining a separate flag is not an issue - we do set instrumentation only on first write attempt and base library also inspects the payload. Actually, maintaining a separate flag provides an ability in base library to avoid log entries payload inspection once instrumentation is written, so we do save cycles.

lgtm. we will need to add an integration test that validating sending a single instrumentation entry when the app uses both logback and JUL and/or explicit log writing.

Yep, integration tests should be added - will open a work item to work on this.