[Metricbeat]Diskio metricset - some windows metric values expressed in increments of 100 nanoseconds

[narph]

For the system/diskio metricset some of the metric values for windows are expressed in increments of 100 nanoseconds instead of miliseconds as stated in the docs (https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntdddisk/ns-ntdddisk-_disk_performance).
Options here are to:

• update the docs and inform users about the differences between the os's
• convert values to milliseconds
  I tend to go for the second one, what do you think @andrewkroh ?

[andrewkroh]

Normalizing to a consistent unit in Beats would be good for usability. So +1 for going to milliseconds since Linux is already reporting in that unit (I'm assuming the latter part about Linux).