[Metricbeat] Expose/export more detailed iostat information

[danleerunk]

Currently, Metricbeat produces system.diskio.iostat.await, but the average can hide important performance details. More useful would be to get r_await and w_await separately, which would allow for better understanding of the nature of disk performance - as well as troubleshooting/alerting on issues where one or the other value is unexpected.

So the idea would be to export those two values, perhaps as system.diskio.iostat.r_await and system.diskio.iostat.w_await. With those two values, system.diskio.iostat.await would no longer be truly required as the end user could produce that value themselves if required.

[ruflin]

As #8508 was merged, can we close this issue? @sayden ?

[sayden]

Yes, thank you for the thumbs up @ruflin