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

@@ -5,7 +5,7 @@ import collectd
 
 PLUGIN = 'btrfs'
 TYPE_STATS = "device_stats"
-
+INTERVAL = 600
 
 def read_callback():
     """Read Btrfs device information and dispatch values to collectd."""
@@ -22,13 +22,17 @@ 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])
+                        metric.dispatch(TYPE_STATS, [value], interval=INTERVAL)
         except Exception as e:
             collectd.error("btrfs: read_callback: {}".format(e))
     collectd.debug("btrfs: read done")
 
 if __name__ != "__main__":
     # Register callbacks
-    collectd.register_read(read_callback)
+    collectd.register_read(read_callback, INTERVAL)
+   
+