[improvement] Add AsyncTracer that tracks enqueued-time and run-time of a deferred action

[robert3005]

Before this PR

DeferredTracer only preserves traceid and last span id

After this PR

Add AsyncTracer that preserves full span stack of parent trace

cc @ellisjoe

[carterkozak]

Why is it important that we can start a span, then complete it on multiple threads? tracing-java makes assumptions about threads that become foot-guns when applied to asynchronous usage. It might be time for a v2 API.

[ellisjoe]

Yeah, we're explicitly interested in tracing async things (eg: how long tasks spend in queues or waiting for their client side limiters)

[carterkozak]

Can we produce a new function for this case with fewer sharp edges? I'm worried about queueing multiple operations, and having them all complete copies of the same span. We need some way to enforce the concept of span ownership.

[robert3005]

@cakofony what is the case you currently have that you're worried about? Isn't it the case that multiple threads can finish copies of the span before #97

[carterkozak]

Yes, that was a bug and a foot-gun.

Perhaps this behavior should live in a separate @Beta tracer type which doesn't need to be serializable. It's not clear to me that DeferredTracer is the right home for this functionality. We don't want to defer a trace, we're elongating a span.

[carterkozak]

If we go with this approach it will require a serialVersionUID bump

[pkoenig10]

Forwarding @dfox's comment about the default operation name: #110 (comment)

[dansanduleac]

I missed this sorry!

[carterkozak]

Rough notes on the detached span idea so I remember later:

/* Does not modify thread state, the current span is used as detached spans parent * if it exists, otherwise a detached span is created with a new trace that has no * thread state. */DetachedSpandetached = Tracer.startDetachedSpan("operation")

interfaceDetachedSpan {
/* Executes the Runnable with tracing state applied. */voidrun(Stringoperation, Runnablerunnable)
/* Executes the Callable with tracing state applied. */
<T> Tcall(Stringoperation, Callable<T> callable)
/* Complete this span */voidclose();
}

[carterkozak]

Thanks @robert3005 👍

[pkoenig10]

Given that this is a new class, we should have made the operation required.