diskstats: add fallback to ID_SERIAL for virtio devices

Virtio devices don't have ID_SERIAL_SHORT but do have ID_SERIAL.
This change adds a fallback to use ID_SERIAL when ID_SERIAL_SHORT
is not available, allowing serial numbers to be properly exposed
for virtio block devices.

Signed-off-by: Harikrishnan R <me+gh@illustris.tech>

Author: illustris

collector/diskstats_linux.go

@@ -60,6 +60,7 @@ const (
 	udevIDModel                 = "ID_MODEL"
 	udevIDPath                  = "ID_PATH"
 	udevIDRevision              = "ID_REVISION"
+	udevIDSerial                = "ID_SERIAL"
 	udevIDSerialShort           = "ID_SERIAL_SHORT"
 	udevIDWWN                   = "ID_WWN"
 	udevSCSIIdentSerial         = "SCSI_IDENT_SERIAL"
@@ -294,6 +295,11 @@ func (c *diskstatsCollector) Update(ch chan<- prometheus.Metric) error {
 			serial = info[udevIDSerialShort]
 		}
 
+		// If still undefined, fallback to ID_SERIAL (used by virtio devices).
+		if serial == "" {
+			serial = info[udevIDSerial]
+		}
+
 		queueStats, err := c.fs.SysBlockDeviceQueueStats(dev)
 		// Block Device Queue stats may not exist for all devices.
 		if err != nil && !os.IsNotExist(err) {