Signals in /metrics and on the dashboard; failed Jev calls are failed spans (#270)

The judge had spans but nothing in Prometheus: no count of items judged or
of how calls to Jev ended, nor whether the judge was still passing. The jev
span recorded an error field without the span's status, so Tempo's failed
traces left Jev out, and a timeout recorded nothing. Calls now send
traceparent like every other call out of ipx.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ipx/CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `/metrics` counts Signals: items judged and fitting, calls to Jev by how they ended and the time they took, time since the judge's last pass, and keys refused; a failed call to Jev shows as a failed trace (#270).
+
 ## [1.3.0] - 2026-10-16
 
 ### Added