Better reporting of total/free usable capacity of the cluster (#15230)

The current code uses approximation using a ratio. The approximation 
can skew if we have multiple pools with different disk capacities.

Replace the algorithm with a simpler one which counts data 
disks and ignore parity disks.

cmd/metrics.go

@@ -577,7 +577,7 @@ func storageMetricsPrometheus(ch chan<- prometheus.Metric) {
 			"Total usable capacity online in the cluster",
 			nil, nil),
 		prometheus.GaugeValue,
-		GetTotalUsableCapacity(server.Disks, s),
+		float64(GetTotalUsableCapacity(server.Disks, s)),
 	)
 	// Report total usable capacity free
 	ch <- prometheus.MustNewConstMetric(
@@ -586,7 +586,7 @@ func storageMetricsPrometheus(ch chan<- prometheus.Metric) {
 			"Total free usable capacity online in the cluster",
 			nil, nil),
 		prometheus.GaugeValue,
-		GetTotalUsableCapacityFree(server.Disks, s),
+		float64(GetTotalUsableCapacityFree(server.Disks, s)),
 	)
 
 	// MinIO Offline Disks per node