tracing integration

[ilslv]

I believe we can support better logging by integrating support for brilliant tracing crate, as current solution described in #177 is quite unintuitive and requires additional setup.

We can implement custom tracing::Subscriber and pass Feature, Scenario and Step context inside tracing::Span. So we can retrieve this information and pass logging event as a new kind of cucumber::Events for logging them, when appropriate time comes.

Unresolved questions, that require additional investigations:

1. spawning a future, that will log after a Step has been finished may lead to strange output behaviour

#[given(...)]
fn delayed_log(w: &mut World) {
    spawn(
        async {
            sleep(Duration::from_secs(1).await;
            tracing::info!("Delayed");
        }
        .instrument(tracing::Span::current())
    )
}

Moreover, spawned Futures by default don't inherit Span from context they were spawned from. See tokio-rs/tracing#394 for more info.

2. Possible integrations with existing tracing ecosystem.

[tyranron]

@ilslv for the reasoning it's slightly unclear what challenges/problems it allows us to solve? What we will be capable with it rather than without it?

[ilslv]

@tyranron this will allow to log something in colored mode and with concurrent execution. Logs captured during some Step execution will be printed only when this Step is actually outputted to the terminal.
This is especially useful, when debugging an error. For now, you have to set concurrent=1 and color=never to actually get the idea, when the particular log occurred. And if reproducing an error requires concurrent execution of multiple Scenarios, you are stuck figuring out, which logs correspond to failed Scenario.

[tyranron]

@ilslv seems reasonable. I think we should give it a try at least.