Enable overriding `target` field with `log`'s target in `LogTracer`

Feature Request

Crates

tracing-log

Motivation

While the LogTracer transmits log events, the target field is always set to log:, so the original location gets obfuscated.

Here's a minimal example:

Cargo.toml

[package]
name = "trace-log-target"
version = "0.1.0"
edition = "2021"

[dependencies]
datafusion = "34.0.0"
tokio = "1"
tracing = { version = "0.1", features = ["log"] }
tracing-log = "0.1"
tracing-subscriber = { version = "0.3.18", features = ["json", "env-filter"] }

main.rs

#[tokio::main]
async fn main() {
    // Redirecting log events to tracing
    tracing_log::LogTracer::init().unwrap();

    // Set up the tracing subscriber
    let subscriber = tracing_subscriber::FmtSubscriber::builder()
        .with_max_level(tracing::Level::DEBUG)
        //.json()
        .finish();

    // Initialize the tracing subscriber
    tracing::subscriber::set_global_default(subscriber).expect("setting default subscriber failed");

    // Run an external library that uses plain `log`
    let ctx = datafusion::execution::context::SessionContext::new();
    let df = ctx.sql("SELECT 1").await.unwrap();
    df.clone().show().await.unwrap();
}

Example log line with cargo run:

2024-01-25T08:46:42.249999Z DEBUG log: Plan unchanged by optimizer rule 'eliminate_limit' (pass 0)

Note that the target just says log:, which can get pretty confusing when trying to troubleshoot logs from many libraries.

However, if I uncomment the .json() line above I see that the actual target info is still present, albeit nested (datafusion_optimizer::optimizer from the log.target field)

{"timestamp":"2024-01-24T11:26:30.372803Z","level":"DEBUG","fields":{"message":"Plan unchanged by optimizer rule 'eliminate_limit' (pass 0)","log.target":"datafusion_optimizer::optimizer","log.module_path":"datafusion_optimizer::optimizer","log.file":"/Users/gruuya/.cargo/registry/src/index.crates.io-6f17d22bba15001f/datafusion-optimizer-34.0.0/src/optimizer.rs","log.line":309},"target":"log","threadName":"tokio-runtime-worker","threadId":"ThreadId(11)"}

Proposal

I'd like to be able to configure the LogTracer if possible (or something else) such that the target in the default output format prints out the log.target that is used in the JSON output format instead of just log:, so that I can see something like

2024-01-25T08:46:42.249999Z DEBUG datafusion_optimizer::optimizer: Plan unchanged by optimizer rule 'eliminate_limit' (pass 0)

tokio-rs / tracing