Add ability to aggregate disk metrics

[rezib]

The disk plugin generates metrics with multiple DS and aggregation plugin is not able to merge metrics with multiple DS. Then, it is not possible to aggregate metrics of the disk plugin. This is particularly annoying for nodes that have tons of disks, such as server nodes in a distributed filesystems.

This should be fixed in either way:

• split multiple read/write DS in disk plugins into multiple metrics
• make aggregation plugin able to aggregate metrics with multiple DS