Fix missing device name

- now is "missing" instead of an empty string
- this is triggered e.g. when mounting degraded mode. In normal
  operation it shouldn't happen
- Fixes #1

btrfs_plugin.py

@@ -22,6 +22,8 @@ def read_callback():
                             r'[^a-zA-Z0-9]',
                             r'-',
                             dev_info.path)[1:]
+                    if not metric.plugin_instance:
+                        metric.plugin_instance = "missing"
                     for counter, value in stats.counters.items():
                         metric.type_instance = counter
                         metric.dispatch(TYPE_STATS, [value], interval=INTERVAL)