tracing: track goroutine ID in Span

[tbg]

Is your feature request related to a problem? Please describe.
While Span can cross goroutine boundaries, idiomatically it should not. This gives us a straightforward way to, in many cases, heuristically find the goroutine associated to a long-running trace span. Note that the tracked goroutine may already have terminated, and the goroutine ID may now be reused by a different goroutine. This is why it says "heuristically" - in practice this should work well.

Describe the solution you'd like

We grab the goroutine ID (goid package does it) at span creation time (we could also update it on every access, but not sure this buys us much) and can expose it as part of #55733.

Describe alternatives you've considered

#58609 suggests allowing to hook into the output of a span (making it verbose if need be). However, that gives no clue as to the location of a truly stuck span (say a goroutine sitting on a select). It also involves more complexity.

Additional context

We are at the moment targeting "expert debugging" for this use case only. Chasing down a goroutine in this way will be too complicated for end users, but should be within the realm of engineers, TSEs, CC-SREs, CS, etc with proper playbooks.

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