Add "traceSampled" to MDC in the Slf4JEventListener

[jamesmoessis]

This resolves #755

As mentioned this is useful for filtering by looks which will also appear in the tracing tool.

API is backwards compatible and tests are added.

Put "since 1.4.0" on the new constructor, assuming this will be part of next minor release. Happy to change that.

[jamesmoessis]

Closing this as the maintainers saying they are working on a broader related feature set:

The behavior of not adding the sampling flag to the MDC dates back to Sleuth. It used to add it out of the box but it was removed I think because of performance reasons. With Brave, it should be possible to register a “customizer” where you can add any fields you want to the MDC from the current span. We think having the sampling flag in the logs is a useful feature and we are going to look into doing something similar to the Brave “customizer” in the OTel bridge so that we can cover a more general use-case and users can decide what they want to add to the MDC.

If anyone runs into this problem you can extend Slf4JEventListener with this functionality and add it as a bean.