Backport local recorder lifetime casting changes to 0.22 and 0.23.

[tobz]

Context

In #564, a rust-lang team member submitted changes to update a suboptimal instance of implicit lifetime casts that we use in LocalRecorderGuard<'a> related to change they were hoping to make upstream which would potentially turn such a usage into an error.

They mentioned that it would be useful to backport these changes as a patch release to older versions of metrics, given the notion that users on such versions might not be able to trivially update their metrics usage, but in backporting the change, it would ease the pain associated with making the aforementioned code pattern become an error.

It's pretty trivial to create backport branches, and the code involved is straightforward, so we should probably just do it once #564 is merged.

[tobz]

This is done as of [email protected] and [email protected].