make servlet3 + spring webmvc + jaxrs 2.0 indy compatible

[SylvainJuge]

part of #11457

Introduces IndyProxy interface for the classes that are injected into instrumented classes classloader which allows to transparently unwrap the proxy delegate when needed with IndyProxyHelper. This interface and its only method is hidden from the reflection API.

[laurit]

Imo instead of doing the reflection here there should be an API to do this. Something that would unwrap the object when it is proxy and return the input when it is not. So you could write OpenTelemetryHandlerMappingFilter filter = = (OpenTelemetryHandlerMappingFilter) unwrap(springCtx.getBean("otelAutoDispatcherFilter"));
I guess using the reflection as currently is fine for start. Instead of making the delegate field public I'd probably try to make the proxy instance implement an interface that allow unwrapping. If needed that interface could be hidden from reflection

opentelemetry-java-instrumentation/instrumentation/internal/internal-reflection/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/internal/reflection/ReflectionHelper.java

Line 59 in 647fc45

public static Class<?>[] filterInterfaces(Class<?>[] interfaces, Class<?> containingClass) {

Idk how hard all of this would be to implement.

[SylvainJuge]

Hi, I've managed to implement this by introducing an IndyProxy interface that allows us to easily unwrap the proxies when needed with an IndyProxyHelper class. It took me a while and it's definitely a bit more complicated than a public field, but it's way more elegant to use. Also it was a very good way to learn about "how to hide things from reflection API" :-).

Let me know if that's close to what you had in mind here. One thing that is still missing though is properly testing hiding proxy method and interface in the reflection instrumentation.

[SylvainJuge]

Update: tests have been added so it should be ready for review now.