sp-utils: tracing bounded channels

[sorpaas]

rel #6563

This adds bounded channel support for tracing channels, modeled similar to current unbounded tracing channel implementations.

[gnunicorn]

Code-wise looks good to me, just trying to understand the rational (as this PR doesn't contain any case of it being used): with a channel that applies back-pressure, we can't really leak there. What is the intention of tracing the count of their messages?

[bkchr]

Yes, I have the same question. What is the point of tracing a bounded channel?

[sorpaas]

I just thought it's good to have the consistency -- with all our unbounded channels being traced, maybe it's good to have bounded ones traced as well.

Maybe I'm entirely wrong, so not sure if we want to proceed with this PR. If not, let me update #6563 directly using bounded channels.

[gnunicorn]

@sorpaas yeah, consistency is good, but in this case, the error is that we have unbounded channels. Bounded channels are fine and no source of leaks and if we had an entirely async system, we could just use them instead. Adding tracing was a patch to the situation being broken and making sure we don't leak.

IMHO we should focus on replacing the occurrences and get rid of the patch, rather than adding on it. If it is okay with you, I'd rather not merge this...