feat: add more host metrics

[dobrac]

Add metrics per host for:

• file descriptors
• context switches
• threads count

---

Note

Enables process metrics and aggregates them per node into host-level metrics for open file descriptors, context switches, and thread count.

• Observability (otel-collector) in iac/provider-gcp/nomad/configs/otel-collector.yaml:
  • Receivers:
    • Enable process receiver (with mute_process_all_errors: true) and collect selected metrics: process.open_file_descriptors, process.context_switches, process.threads.
  • Processors:
    • Add metricstransform/aggregate_process_metrics to rename and aggregate per node.id:
      • process.open_file_descriptors → system.open_file_descriptors (sum)
      • process.context_switches → system.context_switches (sum)
      • process.threads → system.threads (sum)
  • Pipelines:
    • Update metrics/host to include metricstransform/aggregate_process_metrics alongside existing processors.

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[Copilot]

@dobrac I've opened a new pull request, #1617, to work on those changes. Once the pull request is ready, I'll request review from you.