How to extract the specific field from JSON log saved in field message using Logback and Logstash?

Question

I'm using Zalando Logbook to generate JSON log messages in my Java application. The log messages are configured with a LoggingEventCompositeJsonEncoder in Logback. Now, I want to extract the "custom_field" field from the log message, which is a JSON object, and send it to Logstash.

Here's a snippet of my Logback configuration:

<encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
    <providers>
        <!-- Other providers -->
        <pattern>
            {
                "app_name": "${APP_NAME}",
                "custom_field": "%message{jsonPath='$.custom_field'}"
            }
        </pattern>
    </providers>
</encoder>

The above example dosn't work. The log messages generated by Zalando Logbook are in JSON format, and I want to extract the "custom_field" field from these messages and include it in the Logstash index.

How can I achieve this? Is there a way to extract the "duration" field from the JSON log message and include it in the Logstash index using the LoggingEventCompositeJsonEncoder pattern configuration?