register Oximeter metrics for local disks

[ahrens]

This will cause metrics to be reported for local disks (e.g. reads, writes, etc).

Tested by specifying a UUID in the instance spec toml file, and retrieving the metrics with curl, e.g.:

# Disk in slot 3
[block_dev.4e523bc0-cbaa-4b0e-8461-b7ebaa1053d4]
type = "file"
block_size = 4096
path = "/dev/zvol/rdsk/oxp_03/nocrypt/dump"
workers = 30

[dev.block3]
driver = "pci-nvme"
block_dev = "4e523bc0-cbaa-4b0e-8461-b7ebaa1053d4"
pci-path = "0.8.0"

$ curl http://0.0.0.0:37058/79c39656-34e2-4551-86a5-e94232bf6f06 | jq
...
  {
    "status": "ok",
    "info": [
      {
        "measurement": {
          "timestamp": "2026-05-20T18:00:42.931189546Z",
          "datum": {
            "type": "cumulative_u64",
            "datum": {
              "start_time": "2026-05-20T17:41:28.504981529Z",
              "value": 569 # <== number of read requests
            }
          }
        },
        "timeseries_name": "virtual_disk:reads", # <== metric name
        "timeseries_version": 1,
        "target": {
          "name": "virtual_disk",
          "fields": {
...
            "block_size": {
              "name": "block_size",
              "value": {
                "type": "u32",
                "value": 4096
              }
            },
            "disk_id": {
              "name": "disk_id",
              "value": {
                "type": "uuid",
                "value": "4e523bc0-cbaa-4b0e-8461-b7ebaa1053d4". # <=== matches block_dev UUID
              }
            },
...
          }
        },
        "metric": {
          "name": "reads",
          "fields": {}
        }
      },