feat(openmetrics): add VDI disk size and physical usage metrics

[mpiton]

Description

Add per-VDI disk size metrics to the OpenMetrics/Prometheus endpoint.

Two new gauge metrics are exposed for each VDI:

Metric	Description
xcp_vdi_virtual_size_bytes	Virtual size of the VDI in bytes
xcp_vdi_physical_usage_bytes	Physical space allocated by the VDI on the SR in bytes

Labels: pool_id, pool_name, sr_uuid, sr_name, vdi_uuid, vdi_name, and optionally vm_uuid, vm_name when the VDI is attached to a VM.

The implementation follows the existing SR_METRICS IPC pipeline pattern: data collection in the parent process (#getVdiData()), IPC transport (GET_VDI_DATA/VDI_DATA), formatting in the child process (formatVdiMetrics()), and integration into the /metrics output.

Fixes XO-1995

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes XO-1995)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft

Review process

If you are an external contributor, you can skip this part. Simply create the pull request, and we'll get back to you as soon as possible.

This 2-passes review process aims to:

• develop skills of junior reviewers
• limit the workload for senior reviewers
• limit the number of unnecessary changes by the author

1. The author creates a PR.
2. Review process:
   1. The author assigns the junior reviewer.
   2. The junior reviewer conducts their review:
      • Resolves their comments if they are addressed.
      • Adds comments if necessary or approves the PR.
   3. The junior reviewer assigns the senior reviewer.
   4. The senior reviewer conducts their review:
      • If there are no unresolved comments on the PR → merge.
      • Otherwise, we continue with 3.
3. The author responds to comments and/or makes corrections, and we go back to 2.

Notes:

1. The author can request a review at any time, even if the PR is still a Draft.
2. In theory, there should not be more than one reviewer at a time.
3. The author should not make any changes:
   • When a reviewer is assigned.
   • Between the junior and senior reviews.

[plane-sync-vates]

Linked to Plane Work Item(s)

• [XO-1995] [OpenMetrics] Add VDI Disk Size & allocated on VM

This comment was auto-generated by Plane

[b-Nollet]

Is this necessary to get all the objects?
It would feel more optimal to only get the ones we'll need, which are SRs and VBDs

[mpiton]

Replaced the unfiltered getObjects() with specific filtered queries for SRs and VBDs. The SR lookup now goes through allSrs[vdi.$SR] directly and VBDs through allVbds[vbdId].