Drop-in Plug for bare Plug apps (no Phoenix). Add one line to your pipeline:
plug MetrixWire.Plugand every request opens exactly one trace, with Ecto/Finch spans attaching
automatically via the process dictionary. Phoenix apps need NO such line —
Phoenix emits [:phoenix, :endpoint, :*] telemetry which the SDK already hooks.
This module is Plug-compatible without a compile-time dependency on :plug:
it implements init/1 and call/2 structurally, and it registers a
before_send callback so the trace closes with the final response status.
Defensive at every step — instrumentation must never change the response.