[Observability] Workflow metrics: gateway endpoint and dashboard panel

### Feature Summary
Workflow metrics cannot be viewed in the dashboard. This adds metrics retrieval through the gateway plus an ECharts-based UI panel.
### Proposed Solution or Design
- Backend: a metrics query builder and `parseMetrics`; a `MetricsResource` serving a closed allowlist of named queries (throughput, outcome rates, and duration percentiles), enforced on both the gateway and the client.
- Frontend: a metrics panel rendering each named query as a typed series with Apache ECharts. Chart data is bound as typed arrays; backend output is never interpreted as a formatter or template.
### Affected Area
Workflow UI, Other
### Related
Part of #4070. Depends on #5371.