A way to force a particular a span! to be always enabled regardless of the rest of the configuration

[MOZGIII]

Feature Request

There's no way to guarantee that a particular span is enabled. We'd like to have the ability to force-enable the Span, regardless of the current log level.

Crates

tracing = "0.1.15"
tracing-subscriber = "0.2.2"
tracing-log = "0.1.0"

Motivation

I want to:
a) use tracing::spans fields as labels at metrics crate, see metrics-rs/metrics#87;
b) be able to test this integration.

For both use cases, it'd be pretty good to have the ability to create a span even when the env filter is set to off.

In our test code, when log level is set to off (which is the default for tests), the Span is always disabled:

#[test]
fn test_labels_injection() {
    trace_init(); // inits the tracing in the test environment, defaults to `off`

    let span = span!(Level::ERROR, "my span", component_name = "foobar");
    if span.is_disabled() {
        panic!("span is disabled");
    }

    // ...
}

More info here: vectordotdev/vector#3888

Proposal

Alter the span! macro in such a way that it's possible to force the actual span being created, instead of it being Span::none().

Alternatives

There is a potential way to create such Span manually, by borrowing Callsites and doing other very implementation-specific tricks, but it's not a maintainable solution.

[jyn514]

How would this interact with STATIC_MAX_LEVEL? To avoid a performance hit, I think the static max level would have to take precedence.

[MOZGIII]

I imagine this as something that goes entirely around the level configurations of any kind. Performance hit or not, this is some functionality that has to be on unconditionally. I think this is the whole point.