Observability

Making systems understandable in production: the three pillars (logs, traces, metrics) and how to implement them in Rust.

Structured logging	tracing crate, spans, fields, subscribers
Distributed tracing	OpenTelemetry, trace propagation, sampling
Metrics	Counters, histograms, gauges — Prometheus, metrics crate
tokio-console	Live view of async task states and poll times