ok risolto.

ho spostato il file sotto la cartella resources, e l'ho modificato così:
codice:
<?xml version="1.0" encoding="UTF-8"?>
<Configuration>

    <Appenders>
        <Console name="console" target="SYSTEM_OUT">
            <JsonLayout complete="false" compact="false">
                <KeyValuePair key="campo" value="valore"/>
            </JsonLayout>
        </Console>
    </Appenders>

    <Loggers>
        <Root level="debug" additivity="false">
            <AppenderRef ref="console"/>
        </Root>
    </Loggers>

</Configuration>