[collectd] threshold checking on interface

[Brett Hawn]

I've setup a interface configuration

<Plugin interface>
   Interface "em0"
   IgnoreSelected false
</Plugin>

and then a threshold

<Threshold>
   <Plugin "interface">
     Instance "em0"
       <Type "if_octets">
         DataSource "tx"
         FailureMax 300000
       </Type>
   </Plugin>
</Threshold>

Ideally, this should alert me (via syslog currently), if the interface 
exceeds 300k octets. However, even when I set this as low as 50 (just to 
verify functionality), it did not spawn an alert. Have I gone wrong with 
this somewhere?