Discussion: Asynchronous Tracing

In implementing Money tracing over Apache's `HttpAsyncClient` (#84) I've been giving a lot of thought to how money tracing could be generalized over asynchronous operations in general. Normally in `TraceAspect` when a method annotated with `@Traced` returns the span is immediately stopped.

A thought I had would be to add a new element `async` of type `boolean` with a default value of `false`. If that element value is set to `true` and the return type of the annotated method is determined to be one of the compatible types then the aspect would change how it advices the method so that the span remains open after the method returns successfully and that a callback would be registered on the return value which would be responsible for closing the span.

```java
@Traced(value = "my-async-method", async = true)
public CompletionStage myAsyncMethod() {

// could be anything asynchronous here, not specifically executor/thread related
CompletableFuture future = new CompletableFuture();
executor.submit(() -> {
// do some work
future.complete("All Done");
});

return future;
}
```

The exact list of compatible return types would depend on the references available. Money currently targets JRE 1.6 which would preclude `CompletableFuture` and `CompletionStage` which were added in Java 8 and the current dependencies don't give us many other options. Normal Java `Future` could be instrumented but it would be tricky as that interface provides no mechanism for registering for completion which would require either wrapping/pointcuts and waiting on a call to `Future#get` or polling `Future#isDone` on a separate thread, neither of which are ideal.

It would require more engineering work but a mechanism could be built on Java 6 Service Loader to register classes that would be used to recognize other asynchronous types. That would allow for separate modules that could be used to instrument Java 8, Guava, Spring, etc.