Add orphan GC sweeper and Prometheus metrics with explicit registration

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

internal/discovery/server.go

@@ -32,6 +32,13 @@ type LeaseStore interface {
 	Counts() map[string]int
 }
 
+// LeaseMetrics counts lease acquisitions by outcome. Implemented by
+// internal/metrics; defined here so this package carries no metrics
+// dependency.
+type LeaseMetrics interface {
+	LeaseRequest(outcome string)
+}
+
 const (
 	defaultAddr     = ":8090"
 	defaultTTL      = 5 * time.Minute
@@ -55,6 +62,8 @@ type Server struct {
 	Token string
 	// MaxLeaseTTL caps requested lease TTLs (default 1h; --max-lease-ttl).
 	MaxLeaseTTL time.Duration
+	// Metrics, when non-nil, counts lease requests by outcome.
+	Metrics LeaseMetrics
 
 	log logr.Logger