[Observability] PR6: metrics end-to-end (gateway + UI panel)

[Ma77Ball]

PR6: Metrics end-to-end

Workflow metrics through the gateway plus an ECharts-based UI panel.

Backend

• Metrics query builder and parseMetrics.
• MetricsResource serving a closed allowlist of named queries (throughput, outcome rates, and duration percentiles), enforced on both the gateway and the client.

Frontend

• 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.

Part of #4070. Stacked on PR5 (#5371).
Branch: obs/pr6/metrics.

[Ma77Ball]

/take

[Ma77Ball]

/parent-issue #4070