How to extract value from LogstashMarker to use in conversionRule? (logstash-logback-encoder 7.3)

[neverovskii]

Hello! I have a specific need to include value from LogstashMarker (in this case ObjectAppendingMarker) that was previously saved with log to JSON output defined via pattern:

<pattern>
    <omitEmptyFields>true</omitEmptyFields>
    <pattern>
        {
        ...
        "message": "%msg",
        "message_detailed": "%msg_detailed",
        ...
        }
    </pattern>
</pattern>

For that need I've defined custom Conversion Rule as follows:

<conversionRule conversionWord="msg_detailed" converterClass="foo.bar.MessageDetailedMarkerConverter"/>

private const val MESSAGE_DETAILED_MARKER = "message_detailed"

class MessageDetailedMarkerConverter: ClassicConverter() {
    override fun convert(event: ILoggingEvent): String? {
        val messageDetailedValue = event.markerList.firstOrNull()?.let { parentMarker ->
            val messageDetailedMarker = parentMarker.iterator().asSequence().firstOrNull { marker ->
                marker is ObjectAppendingMarker && marker.fieldName == MESSAGE_DETAILED_MARKER
            }

            (messageDetailedMarker as? ObjectAppendingMarker)?.getFieldValue().toString()
        }
        return messageDetailedValue
    }
}

But I've encountered the problem that .getFieldValue() is protected for ObjectAppendingMarker and SingleFieldAppendingMarker.

I've decided not to use LogstashMarkersJsonProvider to write that into JSON output because:

1. We have multiple markers for every log and only need to write this singular marker value;
2. I want to ensure that message_detailed is written strictly after message field, without messing with JsonGenerator logic;
3. This may become needed for regular console appender too.

Is there any other type of Marker I can use to easily receive it's value, or any other way to achieve the exact result? From looking at what library provides my guess is that the best course of action is to extend ObjectAppendingMarker/SingleFieldAppendingMarker and open the access to fieldValue and write that object as log marker.