Manual span completion

The guard control parameter can be applied to span macros to bind an identifier in the body of the annotated function for the [Span] that's created for it. This span can be completed manually, changing properties of the span along the way:

#![allow(unused)]
fn main() {
extern crate emit;
#[emit::span(guard: span, "wait a bit", sleep_ms)]
fn wait_a_bit(sleep_ms: u64) {
    std::thread::sleep(std::time::Duration::from_millis(sleep_ms));

    if sleep_ms > 500 {
        span.complete_with(|span| {
            emit::warn!(
                when: emit::filter::always(),
                evt: span,
                "wait a bit took too long",
            );
        });
    }
}

wait_a_bit(100);
wait_a_bit(1200);
}

Event {
    mdl: "my_app",
    tpl: "wait a bit",
    extent: Some(
        "2024-04-28T21:12:20.497595000Z".."2024-04-28T21:12:20.603108000Z",
    ),
    props: {
        "evt_kind": span,
        "span_name": "wait a bit",
        "trace_id": 5b9ab977a530dfa782eedd6db08fdb66,
        "sleep_ms": 100,
        "span_id": 6f21f5ddc707f730,
    },
}
Event {
    mdl: "my_app",
    tpl: "wait a bit took too long",
    extent: Some(
        "2024-04-28T21:12:20.603916000Z".."2024-04-28T21:12:21.808502000Z",
    ),
    props: {
        "evt_kind": span,
        "span_name": "wait a bit",
        "lvl": warn,
        "trace_id": 9abad69ac8bf6d6ef6ccde8453226aa3,
        "sleep_ms": 1200,
        "span_id": c63632332de89ac3,
    },
}

Take care when completing spans manually that they always match the configured filter. This can be done using the when control parameter like in the above example. If a span is created it must be emitted, otherwise the resulting trace will be incomplete.