Add disk name to disk_percent_used metric

Currently, the only way to identify a disk in the disk_percent_used metric is by using the `device` tag, which is the low-level NVME device name like `/dev/nvme1n1 `. This isn't very user-friendly.

This device name can easily be mapped to the disk name in GCP by using the `/dev/disk/by-id` links. For example:
```
readlink -f /dev/disk/by-id/google-logs-disk
/dev/nvme1n1
```