Fix documentation of MDC/NDC for log4j
Make the extra/tag keys public for log4j2
Add a way to use a Marker during tests
Add tests to ensure that the Thread name is provided as an extra
Add test to check that the Marker is provided as a tag
Add test to check that MDC details are available as extras
Add test to check that NDC details are available as extras
Add documentation on tags and extras for log4j2