## Proposed commit message
Add memory pressure PSI metrics to the system.process.cgroup.memory
metricset, complementing the existing CPU and IO pressure metrics.

New fields added under system.process.cgroup.memory.pressure:
- pressure.some.{10,60,300}.pct - Share of time with some tasks stalled
- pressure.some.total - Total some pressure time
- pressure.full.{10,60,300}.pct - Share of time with all tasks stalled
- pressure.full.total - Total full pressure time

Closes #47604

## How to test this PR locally

### 1. Build and Run Metricbeat

```bash
cd metricbeat
go build .
```

### 2. Create Test Configuration

```yaml
metricbeat.modules:
- module: system
  period: 5s
  metricsets:
    - process
  processes: ['.*']
  process.cgroups.enabled: true

output.console:
  pretty: true
```

### 3. Run Metricbeat

```bash
./metricbeat -e -c /tmp/metricbeat-psi-test.yml
```

### 4. Verify Memory Pressure Fields

Look for `system.process.cgroup.memory.pressure` in the output:
```json
"memory": {
  "pressure": {
    "some": {
      "10": { "pct": 0 },
      "60": { "pct": 0 },
      "300": { "pct": 0 },
      "total": 0
    },
    "full": {
      "10": { "pct": 0 },
      "60": { "pct": 0 },
      "300": { "pct": 0 },
      "total": 0
    }
  }
}
```

### 5. Compare Before/After (Optional)
[compare-psi-metrics.sh](https://github.com/user-attachments/files/24191696/compare-psi-metrics.sh)
Use the comparison script to compare output from main vs this PR:

```
compare-psi-metrics.sh
Usage: ./compare-psi-metrics.sh <main-output.ndjson> <pr-output.ndjson>
```

## Related issues

- Requires elastic/elastic-agent-system-metrics#274
- Closes #47604

(cherry picked from commit c3f35a9)

# Conflicts:
#	NOTICE.txt
#	docs/reference/metricbeat/exported-fields-system.md
#	docs/reference/metricbeat/metricbeat-metricset-system-process.md
#	go.mod
#	go.sum
#	metricbeat/module/system/fields.go