Adds ci/otelrecv/main.go — a minimal OTLP HTTP/JSON trace receiver that
listens on a random port (port 0) so parallel runs never collide.
The check-dagger Taskfile task now starts the receiver in the background,
passes the port via a mktemp file, runs dagger with OTEL env vars set,
then prints a per-span timing report on shutdown. Falls back to plain
dagger call when Go is not available (e.g. CI containers without Go).
First run will show raw attribute keys so we can learn Dagger's exact
telemetry format and refine the cached/live detection logic.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
