example: tracing example using is_container_run() gate

A new examples/tracing crate showing the cookbook recipe end-to-end:
a tracing_subscriber installed in main() only when is_container_run()
returns true. Two containers (greet, shout) emit tracing::info! that
flow through the subscriber when dispatched in-pod.

Demonstrates both paths verbatim:
- cargo run -p cargo-athena-example-tracing
    -> emits YAML, no tracing setup runs
- CARGO_ATHENA_TEMPLATE=...-greet cargo run -p ... -- '"athena"'
    -> runs greet in-process; INFO greeting{name=athena} appears

Adds tracing 0.1 + tracing-subscriber 0.3 as direct deps on the
example crate only (not the workspace).

Author: mostlymaxi

Cargo.lock

@@ -0,0 +1,16 @@
+[package]
+name = "cargo-athena-example-tracing"
+edition.workspace = true
+version = "0.0.0"
+license.workspace = true
+publish = false
+
+# Demonstrates `cargo_athena::is_container_run()`: gating in-pod-only
+# setup (here, a tracing subscriber) so it fires only when Argo
+# dispatches a container body in a pod -- not on every local
+# `cargo athena emit` / `ls` / `describe` / `submit` invocation.
+
+[dependencies]
+cargo-athena = { workspace = true }
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }

examples/tracing/Cargo.toml

@@ -0,0 +1,26 @@
+//! Two containers that emit `tracing::info!` records. The subscriber
+//! is installed once in `main.rs`, gated on `is_container_run()` so
+//! it only fires in-pod.
+
+use cargo_athena::{container, workflow};
+
+#[workflow]
+pub fn pipeline() {
+    let s = greet("athena".to_string());
+    shout(s);
+}
+
+#[container]
+pub fn greet(name: String) -> String {
+    // These flow through the subscriber installed in `main.rs` only when
+    // running in-pod. Locally via `cargo athena emit` they never fire,
+    // because the body never executes.
+    tracing::info!(name = %name, "greeting");
+    format!("hello, {name}")
+}
+
+#[container]
+pub fn shout(msg: String) {
+    tracing::info!(msg = %msg, "shouting");
+    println!("{}", msg.to_uppercase());
+}

examples/tracing/src/lib.rs

@@ -0,0 +1,31 @@
+//! Demonstrates `is_container_run()`: gate one-time setup (a tracing
+//! subscriber) so it fires only when Argo is dispatching a container
+//! body in-pod -- not on every `cargo athena emit` / `ls` / `describe`
+//! / `submit` invocation, which would otherwise also spawn this
+//! binary to introspect templates.
+//!
+//! Try it both ways:
+//!
+//!   cargo run -p cargo-athena-example-tracing
+//!     -> emits the WorkflowTemplate YAML; NO tracing setup runs.
+//!
+//!   CARGO_ATHENA_TEMPLATE=cargo-athena-example-tracing-greet \
+//!     cargo run -p cargo-athena-example-tracing -- '"athena"'
+//!     -> runs `greet` in-process; tracing IS initialized, you see
+//!        the `INFO greeting{name=athena}` line.
+
+fn main() {
+    // Only initialize the subscriber in-pod. The returned guard (here
+    // just `()`) drops at the end of main(); if your setup needs
+    // flush-on-exit, return a real Drop type instead.
+    let _gate = cargo_athena::is_container_run().then(|| {
+        tracing_subscriber::fmt()
+            .with_env_filter(
+                tracing_subscriber::EnvFilter::try_from_default_env()
+                    .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new("info")),
+            )
+            .init();
+    });
+
+    cargo_athena::entrypoint!(cargo_athena_example_tracing::pipeline);
+}