[vmware vsphere]: Add Virtual Machine Disk Metrics

### Integration Name

VMware vSphere [vsphere]

### Dataset Name

vsphere.virtualmachine

### Integration Version

1.23.1

### Agent Version

9.2.6

### OS Version and Architecture

Ubuntu 20.04

### User Goal

I’m trying to monitor per‑VM disk usage and disk performance metrics (e.g., disk capacity, used space, free space, read/write IOPS, and latency) directly through the Elastic vSphere integration. This is necessary for identifying storage‑related bottlenecks, forecasting capacity, and correlating VM‑specific disk issues with datastore and host performance.

### Existing Features

The current Elastic vSphere integration collects metrics for:

- datastores (capacity, usage, read/write operations, latency, throughput) [elastic.co]
- hosts (CPU, memory, disk I/O, network metrics) [elastic.co]
- virtual machines (CPU, memory, network, uptime, status) — but not disk usage or disk I/O

Even in the enhanced vSphere integration (v1.15.0+), disk‑related enhancements focus on host disk performance and still do not include VM‑level disk metrics; VM datastreams continue to include only CPU, memory, uptime, and status.
Because of this limitation, Elastic lacks visibility into one of the most important resource domains for troubleshooting: per‑VM disk utilization and disk latency. As a result, it's difficult to answer questions like:

> Which VM is consuming the most datastore space?
> Is a specific VM causing high datastore latency?
> Which VMs are approaching disk‑full conditions?

### What did you see?

When reviewing the integration documentation, I saw that the virtualmachine datastream reports only CPU, memory, uptime, status, and network metrics — with no available disk usage or I/O metrics

### Anything else?

_No response_