Always try to return smartmon_device_info metric (#663)

* Always try to return smartmon_device_info metric

Sometimes the 'model family' field is not returned by `smartctl' because
a disk is not in the disk database for the version of smartmontools
installed on the system.

In those cases, the device model and serial number is still returned (at
least as far as I have observed.

Re-work the logic to prefer the 'vendor' field first, and if not
present, always output a `smartmon_device_info` metric even if some
labels have empty values.

On the box I'm testing this on, where previously no metric was returned,
it now returns:

    # HELP smartmon_device_info SMART metric device_info
    # TYPE smartmon_device_info gauge
    smartmon_device_info{disk="/dev/sda",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdb",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdc",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdd",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sde",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1
    smartmon_device_info{disk="/dev/sdf",type="sat",model_family="",device_model="INTEL REDACTED",serial_number="REDACTED",firmware_version="REDACTED"} 1

* Add trailing newline

Because POSIX:
https://stackoverflow.com/a/729795

text_collector_examples/smartmon.sh

@@ -102,10 +102,10 @@ parse_smartctl_info() {
       esac
     fi
   done
-  if [[ -n "${model_family}" ]] ; then
+  if [[ -n "${vendor}" ]] ; then
-    echo "device_info{disk=\"${disk}\",type=\"${disk_type}\",model_family=\"${model_family}\",device_model=\"${device_model}\",serial_number=\"${serial_number}\",firmware_version=\"${fw_version}\"} 1"
-  elif [[ -n "${vendor}" ]] ; then
     echo "device_info{disk=\"${disk}\",type=\"${disk_type}\",vendor=\"${vendor}\",product=\"${product}\",revision=\"${revision}\",lun_id=\"${lun_id}\"} 1"
+  else
+    echo "device_info{disk=\"${disk}\",type=\"${disk_type}\",model_family=\"${model_family}\",device_model=\"${device_model}\",serial_number=\"${serial_number}\",firmware_version=\"${fw_version}\"} 1"
   fi
   echo "device_smart_available{disk=\"${disk}\",type=\"${disk_type}\"} ${smart_available}"
   echo "device_smart_enabled{disk=\"${disk}\",type=\"${disk_type}\"} ${smart_enabled}"