fix: Prometheus metrics to re-use storage disks (#11647)

also re-use storage disks for all `mc admin server info`
calls as well, implement a new LocalStorageInfo() API
call at ObjectLayer to lookup local disks storageInfo

also fixes bugs where there were double calls to StorageInfo()

cmd/erasure-server-pool.go

@@ -331,6 +331,35 @@ func (z *erasureServerPools) BackendInfo() (b BackendInfo) {
 	return
 }
 
+func (z *erasureServerPools) LocalStorageInfo(ctx context.Context) (StorageInfo, []error) {
+	var storageInfo StorageInfo
+
+	storageInfos := make([]StorageInfo, len(z.serverPools))
+	storageInfosErrs := make([][]error, len(z.serverPools))
+	g := errgroup.WithNErrs(len(z.serverPools))
+	for index := range z.serverPools {
+		index := index
+		g.Go(func() error {
+			storageInfos[index], storageInfosErrs[index] = z.serverPools[index].LocalStorageInfo(ctx)
+			return nil
+		}, index)
+	}
+
+	// Wait for the go routines.
+	g.Wait()
+
+	storageInfo.Backend = z.BackendInfo()
+	for _, lstorageInfo := range storageInfos {
+		storageInfo.Disks = append(storageInfo.Disks, lstorageInfo.Disks...)
+	}
+
+	var errs []error
+	for i := range z.serverPools {
+		errs = append(errs, storageInfosErrs[i]...)
+	}
+	return storageInfo, errs
+}
+
 func (z *erasureServerPools) StorageInfo(ctx context.Context) (StorageInfo, []error) {
 	var storageInfo StorageInfo