BacklogAvailble becomes a negative number

[doodle-tnw]

When reporting backlog available I see a negative number often.

This is both reported through the admin tool and the statsd output. Because the statsd gauge is used to report the metric, the negative number just decrements the previous value rather than setting it to the new negative number - This means within statsd queue backlogs are hard to judge unless there is currently a backlog.