Loggerfields for Syslog (write priority and stack trace)

Question

Log4j can send logs to Papertrail using a syslog appender (documentation):

log4j.appender.syslog=org.apache.log4j.net.SyslogAppender
log4j.appender.syslog.Facility=LOCAL7 
log4j.appender.syslog.FacilityPrinting=false 
log4j.appender.syslog.Header=true 
log4j.appender.syslog.SyslogHost=<host>.papertrailapp.com:XXXXX
log4j.appender.syslog.layout=org.apache.log4j.PatternLayout 
log4j.appender.syslog.layout.ConversionPattern=%p: (%F:%L) %x %m %n

Note the convenient property ConversionPattern.

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I managed to put something similar to work on log4j2:

Syslog:
  name: Syslog
  id: [my-id]
  appName: [my-app]
  protocol: UDP
  format: RFC5424
  facility: local7
  host: <host>.papertrailapp.com
  port: XXXXX
  includeMDC: true
  mdcId: mdc 

This works and writes messages in standard RFC5424 format:

May 14 14:31:36 [server] [app]: Minha mensagem de log

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What I really want is a message with priority information as well as entire Traces stack:

May 14 14:31:36 [server] [app]: ERROR Minha mensagem de log MyException
        at package1.MyClass.method1(MyClass.java:20)
        at ...

It seems that Loggerfields should do this, but I can’t make it work:

 loggerFields:
   LoggerFields:
     KeyValuePair:
       key: priority
       value: "%p"
     KeyValuePair:
       key: exception
       value: "%ex"

How can I make log4j2 write in syslog using the desired format?

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Cross-post: Soen - Loggerfields for Syslog (output log Priority and stack trace)