Make "frame" spans go on a separate trace line than others

[Stypox]

tracing_chrome supports two styles of data that are shown differently in https://ui.perfetto.dev:

• "Threaded", i.e. each thread has its own trace line in the UI, each line with a tree structure of nested spans. Since Miri is single-threaded, this is equivalent to having a single trace line with all of the spans.

• "Async", i.e. each span has its own trace line in the UI (this is also the representation the Firefox profiler uses).

I believe the "Threaded" style generally allows for more insights as argued here, however it's a bit cumbersome to use because the "frame" spans (i.e. those that indicate stack frames in the interpreted program) are very big with respect to other spans and are therefore in the way. So this PR puts them (and only them) on a separate trace line:

Ideally this kind of filtering should be doable from the trace viewer UI, but I don't think https://ui.perfetto.dev supports anything like that.

Let me know if I should make the changes more general, e.g. by adding a new TraceStyle like ThreadedWithExceptions(Vec<&'static str>) instead of a hardcoded if. I don't think we're going to need this for other spans though. <- this was done

[RalfJung]

I'm definitely not a fan of hard-coding a particular name here, that way lies total spaghetti code.

[Stypox]

Makes sense, I've made it more general

[Stypox]

@rustbot ready

[RalfJung]

You just moved the "frame" name checking to a different file, I don't see how this is better.

The "frame" span itself should have appropriate metadata that controls this behavior, rather than special-casing the name after it has already entered the logging system. IOW, the code here is in charge of saying what happens with frame.

[RalfJung]

I don't fully understand neither tracing nor the visualizations you show, but it seems like you might want the perf-related spans in one "line" and the debugging-related ones in another? frame is just an example here for the one debugging span that is actually enabled in regular rustc build, but there is nothing very special about it.