Appender-tracing and target

[cijothomas]

OpenTelemetry-Appender-Tracing is currently ignoring the target from the Metadata. It is stored as an attribute "log.target" when using experimental feature flag.

target from tracing is what OTel calls instrumentation scope's name. Hence, the target should be stored as instrumentation_scope.name.

Currently, the instrumentation_scope is hardcoded to be "opentelemetry-appender-tracing", which makes it less useful, as every LogRecord will have the same information, defeating the purpose of scope/target in OpenTelemetry.

Suggested fixes:

1. Modify the appender to use a different Logger for each target it encounters. This would likely kill perf/throughput due to the need of a read or read+insert to a lock-protected-hashmap in the hotpath. This could be mitigated via thread-local hashmap, so the contention can be solved at the expense of more memory. (IIUC, OTel Java uses this approach, but not sure if they use thread-local optimization or not)
2. Store target as a special attribute _tracing.target in the LogRecord. In the OTLP Exporter thread, specially treat "_tracing.target" and use that to construct instrumentation_scope_name. (and then do not export this special attribute). OTel .NET does something like this to avoid performance bottleneck: [OTLP] Export LogRecord.CategoryName as InstrumentationScope name opentelemetry-dotnet#4941

[stormshield-fabs]

Note that for events, tracing defaults to a target like "event file:line", which does not really fit the OTel semantics for instrumentation_scope.name.

[cijothomas]

Note that for events, tracing defaults to a target like "event file:line", which does not really fit the OTel semantics for instrumentation_scope.name.

I tried the below example, slightly modified from example, and can see that target is coming as "basic", "basic::foo", and "basic::foo::baz" respectively...

//! run with `$ cargo run --example basic

use opentelemetry::KeyValue;
use opentelemetry_appender_tracing::layer;
use opentelemetry_sdk::{
    logs::{Config, LoggerProvider},
    Resource,
};
use tracing::error;
use tracing_subscriber::prelude::*;

use crate::foo::bar;

fn main() {
    let exporter = opentelemetry_stdout::LogExporter::default();
    let provider: LoggerProvider = LoggerProvider::builder()
        .with_config(
            Config::default().with_resource(Resource::new(vec![KeyValue::new(
                "service.name",
                "log-appender-tracing-example",
            )])),
        )
        .with_simple_exporter(exporter)
        .build();
    let layer = layer::OpenTelemetryTracingBridge::new(&provider);
    tracing_subscriber::registry().with(layer).init();

    error!(name: "my-event-name", event_id = 20, user_name = "otel", user_email = "otel@opentelemetry.io");
    foo::bar();
    foo::baz::qux();
    drop(provider);
}

mod foo {
    use tracing::error;

    pub fn bar() {
        error!(name: "my-event-name", event_id = 21, user_name = "otel");
    }

    pub(crate) mod baz {
        use tracing::error;

        pub fn qux() {
            error!(name: "my-event-name", event_id = 22, user_name = "otel");
        }
    }
}

[cijothomas]

@lalitb Missing target is limiting the utility - please reconsider the priority for this item, and move it to beta, if possible.