Create an abstraction for a LoggingContext containing request properties

Currently, armeria natively exports many useful attributes to logback logs

https://github.com/line/armeria/blob/master/logback/src/main/java/com/linecorp/armeria/common/logback/BuiltInProperty.java

But since these are hard-coded into the logback implementation itself, it can’t be reused for log4j users. Could we add to a RequestContext a layer that accumulates key/value pair that logging layers can read without special processing?

Here is a similar idea, but misses out on armeria’s built-in logging, which would be very tedious to duplicate.

https://github.com/curioswitch/curiostack/blob/master/common/server/framework/src/main/java/org/curioswitch/common/server/framework/logging/RequestLoggingContext.java#L39

[trustin]

IMO core API seems fine since we have many logging features there already, like LoggingService. To be honest, I was surprised to find https://github.com/line/armeria/blob/master/logback/src/main/java/com/linecorp/armeria/common/logback/RequestContextExporter.java in logback package since it doesn’t touch any logback APIs at all and it’s a ton of code to possibly duplicate into log4j implementation.

Creating a shared artifact for that or just duplicating might be fine

Perhaps we could clean up the common export logic a little bit and move it to armeria.common or into a utility method of RequestContext? e.g. Map<String, String> ctx.exportProperties(...properties to export...)

but also I do think there’d be a lot of convenience if we could offer requestCtx.logCtx().put("entityId", id) so it’s very easy for business logic to add additional tags without worrying about threads. Does that make it worth putting it into core?

RequestContext already extends AttributeMap which overlaps with it to some extent. How about replacing AttributMap with our own interface which defines a key which tells if its value needs to be exported to logging framework or to the String representation of RequestContext? e.g.

private static AttributeKey<String> ENTITY_ID =
        AttributeKey.of("entityId", /* exported */ true);
private static AttributeKey<ComplexObject> COMPLEX_OBJECT = 
        AttributeKey.of("...", true, complexObject -> "stringified: " + complexObject.propX());

[trustin]

The groundwork for adding support for Log4J2 is now done.