How do I customize a GELF appender for log4j2? - log4j2

Question

We have a log4j2-graylog.xml file that we use to bridge our log4j output to GrayLog 3.0 using the biz.paluch.logging.gelf.log4j2 package. This config file is based on this example:
<?xml version="1.0" encoding="utf-8"?>
<Configuration monitorInterval="60" packages="biz.paluch.logging.gelf.log4j2">
<Appenders>
<Gelf name="gelf"
host="10.13.10.192"
port="12201"
version="1.1"
extractStackTrace="true"
filterStackTrace="true"
mdcProfiling="true"
includeFullMdc="true"
maximumMessageSize="8192"
originHost="swarm"
additionalFieldTypes="environment=String,application=String">
<Field name="timestamp" pattern="%d{dd MMM yyyy HH:mm:ss,SSS}"/>
<Field name="level" pattern="%level"/>
<Field name="simpleClassName" pattern="%C{1}"/>
<Field name="className" pattern="%C"/>
<Field name="server.fqdn" pattern="%host{fqdn}"/>
<Field name="threadName"/><!-- didn't seem to work -->
<!-- This is a static field -->
<Field name="application" literal="PISM-webapp"/>
</Gelf>
</Appenders>
<Loggers>
<Logger name="com.xxx" level="DEBUG" additivity="false">
<AppenderRef ref="gelf"/>
</Logger>
</Loggers>
</Configuration>
It basically works. Our Java calls to log4j logging show up in GrayLog.
Based on the information from here, it seems like it ought to be easy to add a field that captures the thread data, but our attempts thus far have failed. Also we would like a field that reports the value of a certain environment variable. Can anybody tell me how to do either of these two things?

Answer 1

Based on information from the log4j2 manual, I was able to get the thread name by adding threadName to the additionalFieldTypes and using a %t pattern specifier like so:
<Gelf name="gelf"
host="10.13.10.192"
port="12201"
version="1.1"
extractStackTrace="true"
filterStackTrace="true"
mdcProfiling="true"
includeFullMdc="true"
maximumMessageSize="8192"
originHost="swarm"
additionalFieldTypes="environment=String,application=String,threadName=String,fbHost=String">
...
<Field name="threadName" pattern="%t"/>
However, when I tried to use the syntax provided on the page to retrieve the value of an environment variable like so:
<Field name="fbHost" pattern="$${env:LB_COOKIE_VALUE}" />
I didn't get the value of the LB_COOKIE_VALUE environment value in GrayLog. Instead, I got the literal string - "${env:LB_COOKIE_VALUE}". This StackOverflow post gave enough information to debug the problem. When I provided a default value,
<Field name="fbHost" pattern="$${env:LB_COOKIE_VALUE:-unset_env_LB_COOKIE_VALUE}" />
I got "unset_env_LB_COOKIE_VALUE" in the output, indicating that the environment variable was not set. However, the Java system property was set, so
<Field name="fbHost" pattern="$${sys:LB_COOKIE_VALUE:-unset_sys_LB_COOKIE_VALUE}" />
provided the value I was looking for.