The source code of Mats3 is hopefully quite sanely organized, and it has quite a bit of tests. It is worth checking out the repository and delve into some parts.
Clone down the repository and build it (you can do that in a container to be on the safer side):
git clone email@example.com:centiservice/mats3.git # git clone https://github.com/centiservice/mats3.git cd mats3 ./gradlew clean build
There’s also a very rudimentary “dev area” for the Metrics Interceptor MatsMetrics_TestJettyServer, and same for Local Introspector LocalHtmlInspect_TestJettyServer, both of which you may start from your IDE by Right-click -> debug.
Hitting up http://localhost:8080/