Need some way to enable "tokio=trace" filter for the console Layer only

[sbrocket]

First off...this is super cool, thanks for creating it!

The requirement to enable trace-level events for tokio to use this console means that other layers also receive those events, which makes a fmt Layer pretty unusable without out-of-band filtering.

Per tokio-rs/tracing#302, it looks like per-layer filtering does not yet exist. Before I'd discovered that I'd tried combining separate EnvFilters with a fmt Layer and a console Layer using Layer::and_then, but this clearly doesn't work as expected.

Maybe there's another workaround I haven't thought of?

[hawkw]

For now, the best solution is to use MakeWriterExt to add an output filter that disables tokio=trace to the writer used by your fmt layer. This will prevent it from printing events emitted by Tokio, but still allows the subscriber to record them, so that they can be used for the console.

For example, you could write

use tracing_subscriber::prelude::*;

let fmt = tracing_subscriber::fmt::layer()
    .with_writer(std::io::stdout
        // Don't print logs for events with the `tokio` target.
        .with_filter(|meta| meta.target() != "tokio")
    );
    // ... plus any additional `fmt` configurations you may want...

console_subscriber::build()
    .with(fmt)
    .init();

The one caveat here is that the spans used by the console will not be filtered out when printing events that are enabled by other filter settings, so those spans will be shown as part of the span context for those events. A stopgap solution for until we have proper, generic per-layer filtering could be to add a similar form of filtering to tracing-subscriber::fmt when formatting span contexts, so that the fmt layer could skip over spans that don't match a predicate as it builds a context.

See also #64 (comment)

[hawkw]

upstream change tokio-rs/tracing#1523 should fix this in a somewhat nicer way