Add on_record to support tracing::field::Empty and span.record

[seyyed-dev]

Currently, when tracing::field::Empty is used with span.record() calls, the recorded values are lost and not displayed in span traces. This PR introduces on_record functionality to preserve and display recorded values.

let lazy = span!(Level::DEBUG, "lazy span", work_units = Empty);
lazy.record("work_units", &3);
lazy.in_scope(|| {
    info!("doing some work");
});
drop(lazy);

Before

1:main└─┐open: deferred::lazy span 
1:main  ├─ INFO deferred doing some work
1:main ┌┘close(v): deferred::lazy span 

After

1:main└─┐open: deferred::lazy span work_units=3
1:main  ├─ INFO deferred doing some work
1:main ┌┘close(v): deferred::lazy span work_units=3

[seyyed-dev]

If you review commit by commit, you can see the effect of on_record

[seyyed-dev]

Clippy errors of the main branch are fixed at #90

[oli-obk]

Good catch, thank you! and thanks for the CI fixes in separate PRs

[oli-obk]

published as 0.4.1