rust-observabilitylisted

# Rust Observability A log line is a structured event with named fields, and a library never installs a subscriber. This skill governs *how Rust code reports what it is doing*; what an error type contains is `/rust-errors`, and span behaviour across cancellation is `/async-rust`. ## A log line is an event, not a sentence The core move: fields stay fields. `error!(user_id, %path, "config load failed")` is queryable — `user_id` and `path` are named columns. The interpolated form, `error!("config load failed for user {user_id} at {path}")`, is a string someone will later write a regex against. The difference shows up the first time somebody needs "all failures for one user" — the moment logging either pays for itself or does not. ## The three layers, and which to use `println!` writes to stdout with no level, no filter, and no structure; it belongs in a CLI producing output the user asked for, never in a library. `log` is the older facade, right when the dependency budget is tight. `tracing` is the default here: spans carry context across `.await` points, which is the problem async code actually has. ## Naming events `component.operation.state` — `config.load.failed`, `pool.connection.acquired`. A consistent scheme is what makes a dashboard possible; ad-hoc English messages are what make it impossible. Message templates with placeholder syntax are one implementation of the field idea, not the idea itself — a codebase already on templates is fine. ## Spans `#[tracing::in