**Problem:** Fusion had no Prometheus-scrapeable metrics surface, so
production CPU/health/DB/fleet observability had to be inferred from
logs and ad-hoc monitoring.
**Fix:** Expose a Prometheus-text `/metrics` HTTP endpoint on the
dashboard with runtime + domain samplers: request latency quantiles
(p50/p95/max), last-request-age freeze indicator, CPU user/system,
RSS/heap, child-process spawn rates by kind, git children count, PG tps,
active/idle/total projects, running agents, and board task counts.
Scraped at 5s by the existing Prometheus/Grafana stack
(fusion-observability).
**Includes:** full test coverage for the Prometheus text parser and each
sampler.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added a public, unauthenticated `/metrics` endpoint with
Prometheus-formatted dashboard, runtime, process, database, and domain
metrics.
* Added request latency, resource usage, child-process, project
activity, agent, and task metrics.
* Metrics remain available in headless mode and provide stable snapshots
without triggering additional data writes or on-demand I/O.
* **Documentation**
* Documented metric families, sampling behavior, and degraded-operation
handling.
* **Tests**
* Added comprehensive coverage for collection, formatting, parsing,
endpoint behavior, and sampler lifecycle.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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Co-authored-by: Fusion <noreply@runfusion.ai>
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