Instrument Tokio Wakers about console HOT 4 CLOSED

Yes, by instrumenting wakers and also resources (#39), we should be able to correlate these relationships and show them in the Console.

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I did a rough prototype pass of this a fairly long while ago (which, I think, is probably outdated and won't work with the latest Tokio). One thing I did is add spans to the various resources' poll methods, as well, so that when a waker is cloned, we can record which resource we are registering interest in, and (in the case of resources that have multiple forms of readiness) which form of readiness we are polling for.

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The other way around could also work. Since a "subscriber" already needs to produce a unique span ID, it'd be useful to be able to reuse that than to have a second unique counter. The "other way" would be to save the span ID into Tokio's RawTask struct, so that the waker logs can grab it and emit it.

It does seem like a bit of annoying propagating of an extra argument through all the internal Tokio task creation code, but conditionally (so plenty of cfg_trace). But it seems like the way.

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Done in tokio-rs/tokio#3836, opened #42 for next step.

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