Concept-Check: Telemetry

Kind: Quiz. One pass over the whole arc before you move on.

The Telemetry arc gave the coordinator eyes and a ledger. Spans stamp the run_id onto every line a run produces, so concurrent jobs stop being an indistinguishable pile of strings; #[tracing::instrument] opens that span for you and carries it across every .await; EnvFilter lets an operator dial the detail up or down from RUST_LOG without a recompile; and /stats accounts for tokens and cost per model — derived at read time from the same usage the workers already report, never stored twice. This check mixes compiler-verified aggregation questions with judgment about spans, filtering, and where derived values belong.

If a question stings, the fix is upstream: Concept: tracing for spans, instrument, EnvFilter, and the TraceLayer, and Build: Run Spans and /stats for the aggregation and cost math. Re-read the section, then come back.

Next: Part VIII, the Cluster arc — a framed TCP protocol, a connection actor multiplexing one socket, the RemoteWorker behind the same WorkerHandle trait, and at-least-once redelivery made safe by the store's idempotency. The payoff the whole course was built toward.