Fix the type of the disk IO rate measurements

The type of the following measurements should be Gauge:
disk.write.requests.rate
disk.write.bytes.rate
disk.read.bytes.rate

Closes-bug: #1295088
Change-Id: Ia11f8ca36a02ca2e5f74de275df80faeaa5b7d39
Merge "Fix the type of the disk IO rate measurements"