Derived iostat metrics

[tsuna]

iostat derives a few values from the counters we already collect in iostat.py such as (from the iostat manual):

• await: The average time (in milliseconds) for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.
• svctm: The average service time (in milliseconds) for I/O requests that were issued to the device.
• %util: Percentage of CPU time during which I/O requests were issued to the device (bandwidth utilization for the device). Device saturation occurs when this value is close to 100%.

These are to be taken with a grain of salt (a lot of people really believe that %util accurately reflect the percentage of utilization of the I/O subsystem) but they would be useful to have anyway.

For reference, the code of iostat is here.

[tsuna]

Oops, sorry this was a tcollector bug: stumbleupon/tcollector#10