feat(fastrace-futures): add StreamExt::enter_on_poll

[TennyZhuang]

Adds StreamExt::enter_on_poll to fastrace-futures, mirroring fastrace::future::FutureExt::enter_on_poll.

EnterOnPollStream creates a LocalSpan on every Stream::poll_next(), useful for observing how often a stream is woken.

Closes #172

[tisonkun]

Generally LGTM. Thanks for your contribution @TennyZhuang!

[Copilot]

Pull request overview

Adds stream-level poll instrumentation to fastrace-futures, enabling lightweight per-poll_next local spans similar to fastrace::future::FutureExt::enter_on_poll, primarily to observe wake/poll frequency in long-lived streams.

Changes:

• Add StreamExt::enter_on_poll and EnterOnPollStream adapter to create a LocalSpan on each poll_next.
• Add unit tests verifying span creation, parent/child relationships, and span completion on drop.
• Update dev-dependencies to enable fastrace and run reporter-dependent tests serially.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
fastrace-futures/src/lib.rs	Implements enter_on_poll for streams and adds tests validating behavior and span relationships.
fastrace-futures/Cargo.toml	Adds dev-deps needed for span-reporting tests (fastrace with enable, serial_test).

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[andylokandy]

I think we can avoid adding serial_test here. This adapter can be tested through a local collection path instead of the process-global reporter: start a LocalCollector, poll a simple stream under a local parent, then inspect LocalSpans::to_span_records(...). That would keep the test focused and avoid a new dev dependency.

[TennyZhuang]

Done. serial_test removed; tests now use LocalCollector::start() → collector.collect() → to_span_records(...).

[andylokandy]

Could you mention that this needs an outer in_span or another existing local parent? The example uses the right order, but spelling it out would help avoid .in_span(...).enter_on_poll(...), where the local span is entered before the local parent exists.

[TennyZhuang]

Done. Added an "Important: Local parent required" section that spells out the need for an outer in_span or existing local parent, shows the correct .enter_on_poll().in_span() order, and warns against the reversed order.

[TennyZhuang]

@andylokandy Both review items addressed in the latest push:

1. Removed serial_test — dev-dependency deleted. Tests now use LocalCollector::start() → poll stream → collector.collect() → local_spans.to_span_records(parent_ctx). No global reporter needed.

2. Docs strengthened — Added an "Important: Local parent required" section that explicitly states enter_on_poll needs an outer in_span or pre-existing local parent. It shows the correct composition .enter_on_poll("poll").in_span(span) and a warning block for the wrong order .in_span(span).enter_on_poll("poll").

CI is re-running now. Thanks for the review!

[andylokandy]

@TennyZhuang Thank you!