fix: typo in BucketUsageMetrics group registration in v3 metrics (#20090)

```
curl http://localhost:9000/minio/metrics/v3/cluster/usage/buckets
```

Did not work as documented, due to the fact that there was a typo
in the bucket usage metrics registration group. This endpoint is
a cluster endpoint and does not require any `buckets` argument.

cmd/metrics-v3-cluster-usage.go

@@ -139,7 +139,7 @@ var (
 )
 
 // loadClusterUsageBucketMetrics - `MetricsLoaderFn` to load bucket usage metrics.
-func loadClusterUsageBucketMetrics(ctx context.Context, m MetricValues, c *metricsCache, buckets []string) error {
+func loadClusterUsageBucketMetrics(ctx context.Context, m MetricValues, c *metricsCache) error {
 	dataUsageInfo, err := c.dataUsageInfo.Get()
 	if err != nil {
 		metricsLogIf(ctx, err)
@@ -153,11 +153,7 @@ func loadClusterUsageBucketMetrics(ctx context.Context, m MetricValues, c *metri
 
 	m.Set(usageSinceLastUpdateSeconds, float64(time.Since(dataUsageInfo.LastUpdate)))
 
-	for _, bucket := range buckets {
+	for bucket, usage := range dataUsageInfo.BucketsUsage {
-		usage, ok := dataUsageInfo.BucketsUsage[bucket]
-		if !ok {
-			continue
-		}
 		quota, err := globalBucketQuotaSys.Get(ctx, bucket)
 		if err != nil {
 			// Log and continue if we are unable to retrieve metrics for this

cmd/metrics-v3-handler.go

@@ -163,7 +163,7 @@ func (h *metricsV3Server) handle(path string, isListingRequest bool, buckets []s
 		http.Error(w, "Metrics Resource Not found", http.StatusNotFound)
 	})
 
-	// Require that metrics path has at least component.
+	// Require that metrics path has one component at least.
 	if path == "/" {
 		return notFoundHandler
 	}

cmd/metrics-v3.go

@@ -270,7 +270,7 @@ func newMetricGroups(r *prometheus.Registry) *metricsV3Collection {
 		loadClusterUsageObjectMetrics,
 	)
 
-	clusterUsageBucketsMG := NewBucketMetricsGroup(clusterUsageBucketsCollectorPath,
+	clusterUsageBucketsMG := NewMetricsGroup(clusterUsageBucketsCollectorPath,
 		[]MetricDescriptor{
 			usageSinceLastUpdateSecondsMD,
 			usageBucketTotalBytesMD,