go-observability

# Go Observability — Metrics, Traces, and Correlation Production Go services need at least two always-on signals to be debuggable: **metrics** (aggregated measurements for alerting and SLOs) and **traces** (per-request flow showing where time went). The third deliverable is **correlation**: a P99 metric spike must lead, in one click, to the trace that caused it. > **Logging is not in this skill.** Structured logging with `log/slog`, log levels, and zap/logrus/zerolog migration belong to the **go-logging** skill. This skill only references logs in the context of correlating them with traces. ## Core Rules 1. **A feature is not done until it is observable.** New code MUST export at least: one Counter for operations, one Counter for errors, one Histogram for latency. 2. **Histograms, not Summaries, for latency.** Summaries cannot be aggregated across instances; Histograms support `histogram_quantile()` server-side. 3. **Label cardinality is bounded.** Never put unbounded values (user IDs, full URLs, request IDs) in Prometheus labels. Use route patterns, status classes, method. 4. **Context flows everywhere.** A function that does I/O takes `ctx context.Context` as its first argument. No context = no trace propagation = no correlation. 5. **Record errors on the span.** When a span ends in failure, call `span.RecordError(err)` and `span.SetStatus(codes.Error, ...)`. A green span hides a real failure. 6. **Correlate or it didn't happen.** Inject `trace_id` into logs, attach exe...