First of all, as it looks like you are starting from scratch we'd recommend you to use OpenTelemetry for the data collection instead. You can have a look at our Elastic Distribution of the OpenTelemetry java Agent which is designed to provide the best experience for our Kibana UIs.
Regarding your question about what code exactly is inserted: You can look in the vanilla OpenTelemetry Java Agent for all implementations of the TypeInstrumentation: Those define exactly which methods are instrumented and what code is inserted using the ByteBuddy Advice mechanism.
If you have concerns about bytecode-instrumentation, you can also use the OpenTelemetry SDK and just perform manual instrumentation.