fix: rootdisk detection by not using cached value when GetDiskInfo() errors out (#15249)

GetDiskInfo() uses timedValue to cache the disk info for one second.

timedValue behavior was recently changed to return an old cached value
when calculating a new value returns an error.

When a mount point is empty, GetDiskInfo() will return errUnformattedDisk,
timedValue will return cached disk info with unexpected IsRootDisk value,
e.g. false if the mount point belongs to a root disk. Therefore, the mount
point will be considered a valid disk and will be formatted as well.

This commit will also add more defensive code when marking root disks:
always mark a disk offline for any GetDiskInfo() error except
errUnformattedDisk. The server will try anyway to reconnect to those
disks every 10 seconds.

cmd/metrics-v2.go

@@ -232,6 +232,7 @@ type MetricsGroup struct {
 // to populate new values upon cache invalidation.
 func (g *MetricsGroup) RegisterRead(read func(ctx context.Context) []Metric) {
 	g.metricsCache.Once.Do(func() {
+		g.metricsCache.Relax = true
 		g.metricsCache.TTL = g.cacheInterval
 		g.metricsCache.Update = func() (interface{}, error) {
 			return read(GlobalContext), nil