Inform whether a class targeted by instrumentation was already loaded

[ivantopo]

When attaching instrumentation at runtime via Kamon.init() it could happen that some of the classes that are targeted by instrumentation are already loaded which can lead to half instrumented apps. It would be awesome if when attaching at runtime we could at least capture whether a targeted class has been loaded or not and it it is, either log some warnings or make the information available so that it can be used from the status page.