Track redis queue size as collectd metric

Log shippers dump their messages into redis to wait for a parser to get them into elasticsearch. That queue length is a good metric to monitor - high queue lengths mean it's lagging and under-provisioned, consistently low queue lengths means we might be over-provisioned.

Short-term use case: to see how well a particular cluster design is doing at keeping an empty log queue.

Long-term use case: it could be used as a metric for (auto)scaling - when consistently high, provision another parser node to catch things back up.