Vm DiskLatency metric is a direct metric we get from vSphere, here is you can see it in Vi Client.
And the reason you do not see a separate Read Latency metric is because in our latest R2 update for Veeam Management Pack v7, we've moved several less-frequently used metrics into a ResourceKit, we've done this to reduce the footprint of our Management Pack in SCOM databases and improve overall performance and scalability of our MP.
The Managment Pack in Resource Kit is called High-Granularity MP, if you want to get this metric back, you will need to import this MP and put VMs for which you want this metric into a special group.
You can find details there
Let me know if you have any other questions regarding High-Granularity MP or the way we obtain metrics for vSphere targets.