feat(metrics): add backend monitoring metrics and telemetry

[ghshhf]

Summary

This PR enhances the metrics and monitoring subsystem with additional backend monitoring metrics and improved worker file staging telemetry.

This is the third in a series of split PRs (per maintainer feedback). See #10317 for the overall discussion.

Changes

• Modified: core/http/endpoints/localai/backend_monitor.go

  • Extended with additional monitoring capabilities

• Modified: core/http/endpoints/localai/metrics.go

  • Updated metrics collection endpoints

• Modified: core/services/monitoring/backend_monitor.go

  • Improved worker file staging telemetry

• Modified: core/services/monitoring/metrics.go

  • New metric collectors for backend monitoring

Why Separate?

This is a self-contained, non-controversial change:

• No P2P/auth/cache complexity
• No dead code
• Metrics/monitoring enhancements are low-risk
• Clear use case (better observability)

Follow-up PRs (from #10317)

1. ✅ PR 1: Config YAML endpoints (feat(config): add GET /api/models/config-yaml/:name endpoint #10318)
2. ✅ PR 2: Template/context pipeline (feat(templates): add TemplateLoader for context-aware template evaluation #10320)
3. ✅ PR 3 (this PR): Metrics/monitoring additions
4. PR 4: MCP HTTP API routes/client hardening
5. PR 5: Reasoning parser (with tests)
6. PR 6: Distributed cache (with tests, cache invalidation docs)
7. PR 7: P2P node snapshot + ReplaceNodes (design doc, cancellation safety)
8. PR 8: Realtime ephemeral key (HMAC tests, userID binding)

Testing

• Verify new metrics are exported correctly
• Verify backend monitoring telemetry works
• No regression in existing metrics endpoints

Related Issues

Part of #10317