[collectd] Problem using the write_sensu plugin

[Marc Fournier]

Excerpts from Joshua J. Kugler's message of 2016-02-03 15:27:33 -0900:
> [...]
>     Metrics false
>     Notifications true
>     NotificationHandler "pagerduty"
>     NotificationHandler "irc"

collectd dispatches 2 type of events:
 - metrics ("value of %free memory is N" every few seconds)
 - notifications ("warning: %free memory is below N")

Some write plugins only handle metrics (rrdtool), others only handle
notifications (notify_email). A 3rd category handle both (write_riemann,
write_sensu). Hence the Metrics, Notifications, MetricHandler and
NotificationHandler options to the write_sensu plugin.

According to your config, you've disabled metrics dispatching. The
easiest way to make sure the collectd->sensu link is working would be to
flip the Metrics boolean and add a MetricHandler. Or alternatively,
change thresholds so that notifications get raised. See
collectd-threshold(5).

If you don't want to store the metrics collected at all, you can use a
PostCacheChain to send them all to /dev/null. This should stop the "No
write callback has been registered" log message:

PostCacheChain "PostCache"
<Chain "PostCache">
  Target "stop"
</Chain>

But I agree with you, the plugin could be slightly improved to make it
emit a couple of log messages during initalisation. It isn't obvious to
figure that out currently.

HTH,
Marc