Adds internal/metrics: a private registry with the Go runtime + process
collectors and HTTP RED instrumentation (scimesh_http_requests_total and
request_duration_seconds), labelled by method/status and a normalized route so
per-id paths collapse to {id} and never blow up label cardinality.
/metrics is unauthenticated (like /health) for a Prometheus scraper; the
middleware wraps the whole router so every request is measured once.
Efremenko Arhip
e584cfc481