A scrape-time collector reports scimesh_tasks/jobs/workers gauges keyed by
status, sourced from cheap GROUP BY queries (StatsRepo), zero-filled across all
known statuses so the dashboard shows flat zeros instead of gaps. A failed
query yields no samples for that scrape rather than crashing it.
Metrics is now built in main so the DB-backed collector can be registered
(NewServer takes *metrics.Metrics; nil self-provisions for tests). Grafana
dashboard gains a Domain state row: tasks/jobs/workers by status and a queue-
depth stat.
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