Make `Span` `#[must_use]`

Feature Request

Crates

tracing

Motivation

Entered & EnteredSpan are both must use to avoid them being immediately dropped.

Currently nothing prohibits someone from creating a Span and forgetting to enter it. Creating an unentered Span is much more likely to be an error than deliberate and can cause hard-to-find issues if done accidentally.

Proposal

please revive #3477 or #3234 if possible.

If not, I'm more than happy to open a new PR which addresses any issues with those.