Então pesquisei e obtive a resposta:
<target name="database" type="Database">
<dbProvider>
Npgsql.NpgsqlConnection,Npgsql,Version=3.0.3.0,Culture=neutral,PublicKeyToken=5d8b90d52f46fda7
</dbProvider>
<connectionString>
Server=127.0.0.1;Port=5432;User Id=postgres;Password=postgres;Database=sample;
</connectionString>
<commandText>
insert into system_logging(log_date,log_level,log_logger,log_message,log_machine_name, log_user_name, log_call_site, log_thread, log_exception, log_stacktrace) values(@time_stamp, @level, @logger, @message,@machinename, @user_name, @call_site, @threadid, @log_exception, @stacktrace);
</commandText>
<parameter name="@time_stamp" layout="${longdate}"/>
<parameter name="@level" layout="${level}"/>
<parameter name="@logger" layout="${logger}"/>
<parameter name="@message" layout="${message}"/>
<parameter name="@machinename" layout="${machinename}"/>
<parameter name="@user_name" layout="${windows-identity:domain=true}"/>
<parameter name="@call_site" layout="${callsite:filename=true}"/>
<parameter name="@threadid" layout="${threadid}"/>
<parameter name="@log_exception" layout="${exception}"/>
<parameter name="@stacktrace" layout="${stacktrace}"/>
</target>
