metrics: extract boolGauge to simplify RecordInventory

Resolves the SonarQube go:S3776 cognitive-complexity finding on
RecordInventory by replacing the pvc.Healthy if/else with a boolGauge
helper. Adds tests locking the pvc_backup_health gauge to exactly 1 for
healthy and 0 for unhealthy with unchanged metric identity and labels.

Closes #9

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/server/metrics.go

@@ -162,11 +162,7 @@ func (t *telemetry) RecordInventory(inv api.InventoryResponse) {
 			}
 			reasonLabels["reason"] = reason
 			setMetric(t.pvcBackupHealthReason, reasonLabels, 1)
-			if pvc.Healthy {
+			setMetric(t.pvcBackupHealth, labels, boolGauge(pvc.Healthy))
-				setMetric(t.pvcBackupHealth, labels, 1)
-			} else {
-				setMetric(t.pvcBackupHealth, labels, 0)
-			}
 			if pvc.LastBackupAt == "" {
 				continue
 			}
@@ -325,6 +321,15 @@ func incMetric(target map[string]metricSample, labels map[string]string) {
 	target[key] = sample
 }
 
+// boolGauge converts a boolean state into the Prometheus gauge convention of
+// 1 for true and 0 for false.
+func boolGauge(value bool) float64 {
+	if value {
+		return 1
+	}
+	return 0
+}
+
 func setMetric(target map[string]metricSample, labels map[string]string, value float64) {
 	key := metricKey(labels)
 	target[key] = metricSample{labels: cloneLabels(labels), value: value}

internal/server/metrics_test.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -160,6 +161,55 @@ func TestTelemetryRecordInventoryPopulatesAndResetsMetrics(t *testing.T) {
 	}
 }
 
+func TestBoolGaugeMapsHealthToPrometheusValues(t *testing.T) {
+	if got := boolGauge(true); got != 1 {
+		t.Fatalf("expected boolGauge(true) to be 1, got %v", got)
+	}
+	if got := boolGauge(false); got != 0 {
+		t.Fatalf("expected boolGauge(false) to be 0, got %v", got)
+	}
+}
+
+func TestTelemetryRecordInventoryHealthGaugeIdentity(t *testing.T) {
+	telemetry := newTelemetry()
+
+	telemetry.RecordInventory(api.InventoryResponse{
+		Namespaces: []api.NamespaceInventory{
+			{
+				Name: "apps",
+				PVCs: []api.PVCInventory{
+					{
+						Namespace: "apps",
+						PVC:       "data",
+						Volume:    "pv-apps-data",
+						Driver:    "restic",
+						Healthy:   true,
+					},
+					{
+						Namespace:    "apps",
+						PVC:          "cache",
+						Volume:       "pv-apps-cache",
+						Driver:       "longhorn",
+						Healthy:      false,
+						HealthReason: "stale",
+					},
+				},
+			},
+		},
+	})
+
+	rendered := telemetry.render()
+	for _, line := range []string{
+		"# TYPE pvc_backup_health gauge",
+		`pvc_backup_health{driver="restic",namespace="apps",pvc="data",volume="pv-apps-data"} 1`,
+		`pvc_backup_health{driver="longhorn",namespace="apps",pvc="cache",volume="pv-apps-cache"} 0`,
+	} {
+		if !strings.Contains(rendered, line+"\n") {
+			t.Fatalf("expected rendered metrics to contain %q, got:\n%s", line, rendered)
+		}
+	}
+}
+
 func TestTelemetryRecordB2UsageTracksBucketsAndFallbackTimestamp(t *testing.T) {
 	telemetry := newTelemetry()
 	scannedAt := time.Date(2026, 4, 20, 16, 30, 0, 0, time.UTC)