feat(runtime): export trace spans and queue delay through AndroidTraceSink #168

Context

Validating runtime 0.1.0-ALPHA01 tracing on Android (API 35 emulator, PoC: santimattius/android-coroutine-storm#1). I recorded Perfetto traces for three scenarios (unbounded fan-out, hand-rolled bounded, parallel(concurrency = 4)) using DefaultDispatcherProvider.traced(AndroidTraceSink()).

The structured.<label>.pending / .running counter tracks work as documented and clearly show starvation in the unbounded case (peak pending 1997, ~1.6 s above 100). Three gaps made the traces harder to use than the README suggests.

Findings

1. withTraceSpan names never reach the system trace

AndroidTraceSink emits only slices named <label> or <label>:<CoroutineName>. Spans opened with withTraceSpan("checkout") { } do not appear, and there are no flows linking related segments. As a workaround the PoC adds a CoroutineName per scenario (io:storm-bounded).

2. Queue delay is not exported

Slices carry no args, so queueDelayNanos / queue_delay_ns cannot be queried in Perfetto. pending is the only queueing signal in the trace; actual delay had to come from the app's own instrumentation.

3. running counter semantics

running counts dispatched runnables, not logical task concurrency. With parallel(4) it peaked at 10 while real work never exceeded 4 (a unit test asserts this). It cannot be read as "permits in use". Not a bug, but the README should say so.

Proposal

• Emit withTraceSpan names as slices (or Trace.beginAsyncSection, so spans survive thread hops) and link the segments of a span, so a span is visible end to end.
• Attach queue delay to each slice as an arg, or expose it as a counter, when the sink level allows it.
• Document the running semantics, and the <label>:<CoroutineName> slice naming, in the runtime README tracing section.

Evidence (PoC measurements)

Acceptance criteria

• A withTraceSpan block is visible in a Perfetto trace by name, across thread hops.
• Queue delay is queryable from the trace (arg or counter), gated by trace level.
• README tracing section documents slice naming and running semantics.
• Test or sample trace shows the above on Android.

Related: #167