feat(metrics): add named felis_* Prometheus collectors

Introduce internal/metrics exposing the four metric families spec §23
mandates at minimum: felis_servers_total (gauge by desired state),
felis_start_duration_seconds (histogram with Minecraft cold-start
buckets), felis_image_build_failures_total and
felis_reaper_worlds_deleted_total (counters). Collectors are
package-level vars so any subsystem records without an import cycle;
Register wires them into a prometheus.Registerer and is idempotent.

Wire registration into the operator against controller-runtime's global
Registry, so /metrics on the manager's existing metrics endpoint carries
the felis_* families. Instrument the reaper to increment
felis_reaper_worlds_deleted_total in lockstep with Summary.WorldsReaped,
at the one point a world's PVC has actually been deleted.

cmd/felis/operator.go

@@ -6,12 +6,14 @@ import (
 	"io"
 
 	"felis.lolicon.best/internal/apis/felis/v1alpha1"
+	felismetrics "felis.lolicon.best/internal/metrics"
 	"felis.lolicon.best/internal/operator"
 	"k8s.io/apimachinery/pkg/runtime"
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
 	clientgoscheme "k8s.io/client-go/kubernetes/scheme"
 	ctrl "sigs.k8s.io/controller-runtime"
 	"sigs.k8s.io/controller-runtime/pkg/cache"
+	ctrlmetrics "sigs.k8s.io/controller-runtime/pkg/metrics"
 	metricsserver "sigs.k8s.io/controller-runtime/pkg/metrics/server"
 )
 
@@ -57,6 +59,16 @@ func cmdOperator(args []string, _, stderr io.Writer) int {
 	}
 	fmt.Fprintf(stderr, "felis operator: watching namespace %q\n", *namespace)
 
+	// Publish the named felis_* metrics (spec §23) on the endpoint the manager
+	// already serves (metricsAddr). controller-runtime's metrics server exposes
+	// its global Registry, so registering into it is all that is needed for
+	// /metrics to carry felis_servers_total and friends. Register is idempotent,
+	// so an in-process restart never double-registers fatally.
+	if err := felismetrics.Register(ctrlmetrics.Registry); err != nil {
+		fmt.Fprintf(stderr, "felis operator: register metrics: %v\n", err)
+		return 1
+	}
+
 	r := &operator.Reconciler{
 		Client: mgr.GetClient(),
 		Scheme: mgr.GetScheme(),