When I first read about the new Observation API I somehow got an idea that key-value pairs that I would add to the context (e.g. via highCardinalityKeyValues/lowCardinalityKeyValues) would then be automatically set on the baggage fields that I registered (e.g. via management.tracing.baggage.correlation.fields). This would basically ensure that if I were to register a custom baggage field such as userId and report a value for this key in, say, the implementation of ObservationConvention in use, I would then be able to get this field easily in the MDC context and thus have it attached to all log records printed from within the observation.

After multiple attempts to make this work, I've checked the implementation of TracingObservationHandler.java and the only thing that is happening with the context key-value pairs is that they are used to tag the span.

Looking at the old Spring Sleuth docs, I see that there used to be spring.sleuth.baggage.tag-fields property that would allow to specify which tag fields should be added to baggage, but this is not available in Micrometer Tracing / current Spring Boot as far as I can tell.

Is this something that will be implement by Micrometer / Spring at some point or we should work on some work-arounds on our end?

P.S. Right now we kinda have to work with both Observation API (e.g. to avoid working with Trace/Span directly), and at the same time call tracer.createBaggageInScope to make sure custom attributes end up in the logs (via baggage to MDC propagation).