feat(observability): Phase 6 OTLP tracing + Prometheus /metrics endpoint
- Add HTTP OTLP exporter (opentelemetry-otlp 0.31) via SdkTracerProvider +
BatchSpanProcessor + tracing_opentelemetry layer
- Add Prometheus /metrics handler via metrics-exporter-prometheus 0.13
- Replace slog with tracing throughout: HttpMetrics, TracingSpanMiddleware
- Replace .init() with .try_init() to allow OTLP layer registration after
init_tracing_subscriber()
- otlp.rs: SpanExporter::builder().with_http().with_endpoint(),
Resource::builder().with_service_name(), .with_attribute(KeyValue::new(...))
- prometheus_exporter.rs: install_recorder(), prometheus_handler(),
spawn_http_metrics_poller()
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