Fix tracing filtering to allow debug

It seems this code in processes/omnibus/src/main.rs blocks DEBUG output:

```
    let fmt_layer = fmt::layer()
        .with_filter(EnvFilter::from_default_env().add_directive(filter::LevelFilter::INFO.into()))
        .with_filter(filter::filter_fn(|meta| meta.is_event()));
```

`RUST_LOG=debug cargo run --release
`
doesn't produce any debug.