docs: add example for using instrument macro with pinned future

[ziutech]

This change is motivated by a stackoverflow question.

[kaffarell]

Just two small nits:

• I think we can hide the use tracing::Instrument import as well.
• Instead of referring to std::pin, maybe std::pin::Pin (the struct) would be better.

[ziutech]

I think we can hide the use tracing::Instrument import as well.

If you think it's not important I can remove it, but in my opinion it would be better to mention where in_current_span comes from, because this would be the only place the trait Instrument is mentioned on this doc page, and it will be easier to read for someone who doesn't know this trait exists.

Of course if the person reading copy-pastes/duplicates thit example into their code they will get an error to include the trait, so for this case it probably does not matter.

Instead of referring to std::pin, maybe std::pin::Pin (the struct) would be better.

My initial choice for the pin module was that it's "more thorough" (as the docs for std::pin::Pin say), but maybe that does not mean it's better.

Tell me what you think @kaffarell, and I'll change it to whatever you think is better.

[kaffarell]

If you think it's not important I can remove it, but in my opinion it would be better to mention where in_current_span comes from, because this would be the only place the trait Instrument is mentioned on this doc page, and it will be easier to read for someone who doesn't know this trait exists.

Of course if the person reading copy-pastes/duplicates thit example into their code they will get an error to include the trait, so for this case it probably does not matter.

No hard feelings here, so I'm also good with leaving it.

My initial choice for the pin module was that it's "more thorough" (as the docs for std::pin::Pin say), but maybe that does not mean it's better.

Actually std::pin only contains the std::pin::Pin struct (and a macro) and the description is more complete. So I am fine with leaving this as it is as well!

LGTM.