OTel: plumbing (internal/telemetry foundation)

[fabriziosestito]

Part of #972.

Foundation for OTel work: internal/telemetry package wiring Setup, slog trace_id/span_id handler, NATS header carrier, and Tracer/Meter helpers. Called from each cmd/*/main.go. No-op when OTEL_EXPORTER_OTLP_ENDPOINT is unset.