Telemetry API (metrics, tracing, structured logs)

[EdmondDantes]

Expose a first-class telemetry API so operators can observe the server in production.

Scope

• Metrics — counters, gauges, histograms for: accepted connections, active connections per protocol, request rate, request duration, response status classes, bytes in/out, TLS handshake duration, HTTP/2 stream concurrency, HTTP/3 packet rates, CoDel pause events, queue depth.
• Prometheus-compatible text exposition endpoint (opt-in).
• Tracing — OpenTelemetry-compatible spans for accept → parse → dispatch → respond, with context propagation from traceparent header.
• Structured logs — JSON-formatted access log with configurable fields.
• PHP-side hooks: $server->onMetric(), $server->onSpan() so user code can emit custom telemetry on the same pipeline.
• Zero-cost when telemetry is disabled (compile-time and runtime).

Acceptance

• Prometheus scrapes the endpoint successfully and renders the standard server dashboard.
• OTel collector receives spans with correct parent/child relationships.
• No measurable throughput regression with telemetry off; <5% with metrics on.