Add RxJava 3 instrumentation

[amarziali]

What Does This Do

We already trace RxJava 1 and 2 but not 3. RxJava 3 moved its types to the io.reactivex.rxjava3.core.* namespace, so the existing rxjava-2.0 instrumentation never matches it. This adds a new rxjava-3.0 module that brings RxJava 3 to parity.

What's in it:

• New dd-java-agent/instrumentation/rxjava/rxjava-3.0 module. It ports the rxjava-2.0 logic to the RxJava 3 namespace: context capture on the five reactive types (Flowable, Observable, Single, Maybe, Completable) and re-attachment around subscriber callbacks, plus the async result extension that finishes @WithSpan / @Trace spans when the stream completes, errors, or is cancelled.
• Muzzle has a pass for rxjava3 [3.0.0,) and a fail block that asserts the advice can never match the rxjava2 artifact, so the two versions stay isolated. The module also pulls in rxjava-2.0 as a test runtime dependency to confirm both instrumenters coexist.
• Registered in the GraalVM native image build time list next to the rxjava2 entry.
• Tests are JUnit 5 (44 cases): subscription propagation, the core propagation suite, the @WithSpan result extension across all five types, and a Java 8 interop check (fromCompletionStage, fromStream, fromOptional). latestDepTest runs the same suite against the newest published rxjava3.

One thing reviewers should look at closely: this also touches the shared java-concurrent module. While testing delayed chains I found that RxJava 3 added io.reactivex.rxjava3.internal.schedulers.AbstractDirectTask, whose static initializer builds two sentinel FutureTask instances. When that initializer first runs under an active trace (the first delay or timeout hop inside a span), the executor instrumentation captures a continuation on those static singletons that never gets cancelled, so the trace stays pending and is never reported. RxJava 2 has no equivalent class, which is why the byte for byte equivalent rxjava-2.0 code never hit this. The fix disables async propagation while that type initializer runs, following the same pattern already used for Reactor's SchedulerTask and WorkerTask. The matcher is an exact class name, so it stays inert unless RxJava 3 is on the classpath.

The Java 8 interop check found no propagation gaps once that fix was in place.

Motivation

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Jira ticket: [PROJ-IDENT]