Provide Span method `into_enter`

[singpolyma-shopify]

Feature Request

Motivation

It is a common case to create a span, enter it right away, and then never use it again.

Proposal

Provide a method on span into_enter which is the same as the current enter but consumes self rather than taking a reference to it. This would allow the following code:

let _myspan_enter = info_span!("myspan").into_enter();
...

as opposed to the currently required:

let myspan = info_span!("myspan");
let _myspan_enter = myspan.enter();

[hawkw]

Provide a method on span into_enter which is the same as the current enter but consumes self rather than taking a reference to it. This would allow the following code:

let _myspan_enter = info_span!("myspan").into_enter();
...

Unfortunately, the proposed approach won't really work with the current design, since the Entered type holds a borrowed reference to the span. This means that if self is moved into an into_enter method, we can't return an Entered guard pointing to self, as self will only live until the end of that method, and the guard will outlive it.

However, there are a couple of other proposals that would improve ergonomics around a span that's created and immediately entered: #158 proposes a macro that expands to a span and guard, while #79 proposes a way of entering a span that moves rather than borrows the span into the guard. This would also be applicable in use-cases where a span entry should outlive a lexical scope.

An implementation of the former approach was proposed a while ago tokio-rs/tokio#1162; at the time, we closed that branch as it was determined not to be necessary for the particular use-case that originally lead to it being requested (see tokio-rs/tokio#1162 (comment)), but I'd be happy to re-open that PR, as it seems like it might be the best solution to this use-case.

[singpolyma-shopify]

The enter_owned proposal seems to be the closest to what I had in mind in this case, though the macro could also work.