PMM-12962 fix balancer enabled metric (#823)

Author: igroene

exporter/v1_compatibility.go

@@ -1228,24 +1228,20 @@ func balancerEnabled(ctx context.Context, client *mongo.Client) (prometheus.Metr
 		Stopped bool `bson:"stopped"`
 	}
 	var bs bss
-	enabled := 0
+	enabled := 1
 
 	err := client.Database("config").Collection("settings").FindOne(ctx, bson.M{"_id": "balancer"}).Decode(&bs)
-	if err != nil {
+	if err != nil && !errors.Is(err, mongo.ErrNoDocuments) {
 		return nil, err
-	}
-
-	if !bs.Stopped {
-		enabled = 1
+	} else if bs.Stopped {
+		enabled = 0
 	}
 
 	name := "mongodb_mongos_sharding_balancer_enabled"
 	help := "Balancer is enabled"
 
 	d := prometheus.NewDesc(name, help, nil, nil)
-	metric, _ := prometheus.NewConstMetric(d, prometheus.GaugeValue, float64(enabled))
-
-	return metric, nil
+	return prometheus.NewConstMetric(d, prometheus.GaugeValue, float64(enabled))
 }
 
 func chunksTotal(ctx context.Context, client *mongo.Client) (prometheus.Metric, error) {