Add support for log4j2 composite configuration

[quaff]

https://logging.apache.org/log4j/2.x/manual/configuration.html#CompositeConfiguration

Here is my use case, I want output logs to ELK in production, so I add extra log4j2-kafka-appender.xml beside log4j2.xml.
I need merge two files without composite configuration supports, which will make configuration redundant.

logging:
  config: classpath:log4j2.xml

---
spring:
  config.activate.on-profile: prod
logging:
  config: classpath:log4j2.xml,classpath:log4j2-kafka-appender.xml

[wilkinsona]

Thanks for the suggestion. Supporting a comma-separated list and turning that into a CompositeConfiguration looks pretty straightforward. One thing that I'm not completely sure about is having logging.config (which is logging system agnostic) support a Log4j2-specific feature. I think it's ok as it's passed into the various logging systems as a string that they can then interpret as needed, but I'd like to see what the rest of the team thinks.

[snicoll]

I am not keen to see such loose use of a property that's logging system agnostic. It's tricky because creating a specific property will create another set of issues.

[philwebb]

We could support a logging.log4j2.config.override property that could be a List of additional configs. This mirrors the override property discussed in the "Composite Configuration" section of the manual.

[GayathiriRuthraiyah]

I will be working on this issue

[rgoers]

I just saw this issue as I noticed the change in the latest source code.

I should point out that Log4j already provided support for this in its log4j-spring-boot module. In addition, the support that was implemented here doesn't support http(s) connections that require authentication which log4j-spring-boot does. I'd be more than happy to be able to remove log4j-spring-boot once Spring Boot provides the equivalent functionality.