Support for MDC when using async logger

[tomaszdomin-arevocom]

Using log log4j 1.2.17 and logstash-gelf 1.11.2

I've configured gelf appender to work with Async appender:

	<appender name="async" class="org.apache.log4j.AsyncAppender">
		<param name="BufferSize" value="500" />
		<appender-ref ref="console" />
		<appender-ref ref="gelf" />
	</appender>
	<root>
		<priority value="info"/>
		<appender-ref ref="async" />
		<!-- for some reason gelf is not handling MDC under async appender -->
	</root>

When using such setup MDC is not passed by to gelf appender.
it works fine when I moved gelf to root section.

	<root>
		<priority value="info"/>
		<appender-ref ref="async" />
		<appender-ref ref="gelf" />
	</root>

I've tried to set on Async Appender level:
<param name="LocationInfo" value="true"/>
It did not help.

[mp911de]

Good catch. The MDC isn't fetched from the LoggingEvent but statically from the MDC class itself. This causes to return always an empty MDC.

[mp911de]

That's fixed now, snapshots (1.11.3-SNAPSHOT) are deployed to oss.sonatype.org.