ios_in_progress should be Gauge instead of Counter

According to http://www.mjmwired.net/kernel/Documentation/iostats.txt#81 the value represents the currently IO operations in flight. So it not a counter. Seems like a copy&paste mistake to me.
If you force the rate type to Gauge in Bosun's UI, the metric suddenly makes sense :)
Merge pull request #144 from lukaspustina/patch-1

ios_in_progress should be Gauge instead of Counter