feat(coordinator): business metrics — jobs/tasks/workers by status

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.

coordinator/internal/metrics/business.go

@@ -0,0 +1,66 @@
+package metrics
+
+import (
+	"context"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Stats is a point-in-time snapshot of the coordinator's domain state: counts of
+// tasks, jobs, and workers keyed by their status. Maps are expected to be
+// zero-filled by the provider so every known status is always present, giving
+// the dashboard flat zero lines instead of gaps.
+type Stats struct {
+	Tasks   map[string]int
+	Jobs    map[string]int
+	Workers map[string]int
+}
+
+// StatsFunc returns the current snapshot. It is called on every scrape, so it
+// must be a cheap aggregate query.
+type StatsFunc func(context.Context) (Stats, error)
+
+// RegisterBusiness registers a collector that reports domain-state gauges
+// (scimesh_tasks/jobs/workers by status) sourced from collect on each scrape.
+// Deriving the gauges at scrape time keeps them fresh without a background
+// goroutine, and a failed query simply yields no samples for that scrape.
+func (m *Metrics) RegisterBusiness(collect StatsFunc) {
+	m.reg.MustRegister(&businessCollector{
+		collect: collect,
+		tasks:   prometheus.NewDesc("scimesh_tasks", "Tasks by status.", []string{"status"}, nil),
+		jobs:    prometheus.NewDesc("scimesh_jobs", "Jobs by status.", []string{"status"}, nil),
+		workers: prometheus.NewDesc("scimesh_workers", "Workers by status.", []string{"status"}, nil),
+	})
+}
+
+type businessCollector struct {
+	collect              StatsFunc
+	tasks, jobs, workers *prometheus.Desc
+}
+
+func (c *businessCollector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- c.tasks
+	ch <- c.jobs
+	ch <- c.workers
+}
+
+func (c *businessCollector) Collect(ch chan<- prometheus.Metric) {
+	// A bounded query so one slow scrape cannot stall Prometheus.
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	s, err := c.collect(ctx)
+	if err != nil {
+		return // no samples this scrape; Prometheus keeps the last value
+	}
+	emit(ch, c.tasks, s.Tasks)
+	emit(ch, c.jobs, s.Jobs)
+	emit(ch, c.workers, s.Workers)
+}
+
+func emit(ch chan<- prometheus.Metric, desc *prometheus.Desc, counts map[string]int) {
+	for status, n := range counts {
+		ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, float64(n), status)
+	}
+}