An authenticated REST request to an unregistered path reached the inner mux, which left Pattern empty, and metrics.Middleware fell back to the raw URL path: any token holder could create one Prometheus series per unique path. The fallback is now the fixed label "unmatched"; real routes keep their pattern. Tests: buildMux with an authenticated random path, and the middleware unit test; the timing test now routes through a ServeMux as production does. Fixes #185. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>