custom_layer.rs

use std::collections::BTreeMap;
use tracing_subscriber::Layer;

pub struct CustomLayer;

impl<S> Layer<S> for CustomLayer
where
    S: tracing::Subscriber,
    S: for<'lookup> tracing_subscriber::registry::LookupSpan<'lookup>,
{
    fn on_new_span(
        &self,
        attrs: &tracing::span::Attributes<'_>,
        id: &tracing::span::Id,
        ctx: tracing_subscriber::layer::Context<'_, S>,
    ) {
        let span = ctx.span(id).unwrap();
        println!("Got new span!");
        println!("  level={:?}", span.metadata().level());
        println!("  target={:?}", span.metadata().target());
        println!("  name={:?}", span.metadata().name());
        let mut fields = BTreeMap::default();
        let mut visitor = JsonVisitor(&mut fields);
        attrs.record(&mut visitor);
        println!("  fields={:?}", fields);
        let storage = CustomFieldStorage(fields);
        let mut extensions = span.extensions_mut();
        extensions.insert(storage);
    }

    fn on_event(&self, event: &tracing::Event<'_>, ctx: tracing_subscriber::layer::Context<'_, S>) {
        let scope = ctx.event_scope(event).unwrap();
        println!("Got event!");
        for span in scope.from_root() {
            let extensions = span.extensions();
            let storage = extensions.get::<CustomFieldStorage>().unwrap();
            println!("  span");
            println!("    target={:?}", span.metadata().target());
            println!("    name={:?}", span.metadata().name());
            println!("    stored fields={:?}", storage);
        }
    }
}

struct JsonVisitor<'a>(&'a mut BTreeMap<String, serde_json::Value>);

impl<'a> tracing::field::Visit for JsonVisitor<'a> {
    fn record_f64(&mut self, field: &tracing::field::Field, value: f64) {
        self.0
            .insert(field.name().to_string(), serde_json::json!(value));
    }

    fn record_i64(&mut self, field: &tracing::field::Field, value: i64) {
        self.0
            .insert(field.name().to_string(), serde_json::json!(value));
    }

    fn record_u64(&mut self, field: &tracing::field::Field, value: u64) {
        self.0
            .insert(field.name().to_string(), serde_json::json!(value));
    }

    fn record_bool(&mut self, field: &tracing::field::Field, value: bool) {
        self.0
            .insert(field.name().to_string(), serde_json::json!(value));
    }

    fn record_str(&mut self, field: &tracing::field::Field, value: &str) {
        self.0
            .insert(field.name().to_string(), serde_json::json!(value));
    }

    fn record_error(
        &mut self,
        field: &tracing::field::Field,
        value: &(dyn std::error::Error + 'static),
    ) {
        self.0.insert(
            field.name().to_string(),
            serde_json::json!(value.to_string()),
        );
    }

    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
        self.0.insert(
            field.name().to_string(),
            serde_json::json!(format!("{:?}", value)),
        );
    }
}

#[derive(Debug)]
struct CustomFieldStorage(BTreeMap<String, serde_json::Value>);