I recommend using the official prometheus exporter provided by the JavaMelody project itself. Please follow the official guide at https://github.com/javamelody/javamelody/wiki/UserGuideAdvanced#exposing-metrics-to-prometheus
This is a prometheus exporter (https://prometheus.io/docs/instrumenting/exporters) for the JavaEE monitoring tool JavaMelody. It was created to get a deeper insight of the data JavaMelody collects, especially over time without the need to change existing application code to full prometheus monitoring.
Instead of using Go or Python this exporter uses Java because you probably already own a Java infrastructure when running JavaMelody so you can deploy this exporter right next to your application.
This exporter uses the JavaMelody lastValue external API to pull data and transforms it to Prometheus gauges. Exporting is done via a simple Java servlet. No rocket science.
The build is maven based so a mvn package will create the war file in the target folder.
Before doing this you have to create your own javamelody.properties file in src/main/resources/otherwise the build will fail with
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce (Custom javamelody.properties was created) on project javamelody-prometheus-exporter: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [Help 1]
Afterwards just deploy the war file on an application server of your choice. (Tested with Tomcat 7 & 8, minimum Java 6)
javamelody.url=https://<server running javamelody instance>
# Basic authentication credentials if necessary
javamelody.basicauth.username=
javamelody.basicauth.password=
javamelody.collector.server=false
# Comma seperated list of applications (<application1>,<application2>)
javamelody.collector.applications=
As an alternative to this exporter, JavaMelody also includes itself an integration for Prometheus.