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

[thorrester]

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