Github user mdvorak commented on the issue:
https://github.com/apache/logging-log4j2/pull/110
@garydgregory You are right, there is already `AdditionalField`, i'll
rename it.
There is however problem with pulling it up to `AbstractStringLayout`,
since its used by many layouts where would be problematic (or impossible) to
use it. Thats why I chose to implement it directly to JsonLayout.---
