reqwest-tracing: add method for handling cancelled requests

[mladedav]

Motivation

When a request is created and possibly partially sent, but the future is dropped before being polled to completion, then on_request_end is not called.

Solution

This adds another method for this scenario so that either on_request_end or on_request_cancelled is always called if the middleware finishes.

The method has a provided implementation so that it is not a breaking change.

This new method is different from the other two in that it does not take &mut Extensions. This is due to technical difficulities as I would need to keep the mutable reference in the drop guard, but at the same time the future that has access to the same mutable reference would be run. I don't think there is a way to solve this without unsafe and raw pointers (or maybe self-referential structs). So I've decided not to provide the extensions instead.

Alternatives

• We can add another case to the Error enum for this and reuse on_request_end. This would also force us to solve the issue around the &mut Extensions so I think this would also require unsafe code.