[genai-ui] Trace/span detail: GenAI metrics tab

Part of #381
Depends on: #389

## What

Add a GenAI metrics tab to the existing trace/span detail view.

- New tab alongside existing span detail panels
- Shows GenAI semantic attributes extracted from the span: token counts (prompt/completion/total), model name, operation type, latency
- Data sourced from Scouter per-span aggregate endpoint via OpsML proxy
- Only shown when span contains GenAI attributes — tab hidden otherwise
- Follows existing trace waterfall UI patterns