Feature Request: Emit cost / premium-request metric via OpenTelemetry (parity with Claude Code's claude_code.cost.usage)

### Describe the feature or problem you'd like to solve

The Copilot CLI's OpenTelemetry export is great for usage analytics, but it emits **no cost or billing metric**. The metrics currently available are:

- `gen_ai_client_token_usage` (input/output tokens, by model)
- `gen_ai_client_operation_duration`
- `github_copilot_agent_turn_count`
- `github_copilot_tool_call_count` / `..._duration`
- `github_copilot_mcp_server_connection_count`

None of these carry a cost figure. As a result, anyone building a cost dashboard has to **re-derive spend on the consumer side** — either by hand-coding per-million-token list prices per model (an API-equivalent *estimate* that is not the actual Copilot bill), or by trying to reconstruct premium-request consumption, which isn't possible from the telemetry because:

- premium requests are billed per **prompt × a per-model multiplier** (overage $0.04/request), not per token;
- `github_copilot_agent_turn_count` has **no model label**, so turns can't be multiplied by the correct per-model multiplier;
- request counts include agentic tool iterations, which inflate any call-count proxy.

So the one thing teams actually want — "how much did this cost / how many premium requests did it consume" — is the one thing the telemetry can't express today.

### Prior art: Claude Code

Claude Code already exports cost directly via OTel ([docs](https://code.claude.com/docs/en/monitoring-usage)):

| Metric | Description | Unit |
| --- | --- | --- |
| `claude_code.cost.usage` | Cost of the Claude Code session | USD |
| `claude_code.token.usage` | Tokens used | tokens |

This makes a "cost per model / per session" dashboard a one-line query with no client-side price tables.

### Proposed solution

Emit a first-class cost/billing metric from the Copilot CLI's OTel exporter, ideally with the same labels already present on the token metric (`gen_ai_request_model` / `gen_ai_response_model`, and `session.id` if available). Any of the following would solve it, in rough order of preference:

1. **`github_copilot.premium_request.count`** — premium requests consumed, labeled by model (this is the true Copilot billing unit). The CLI already knows the prompt boundaries and the model multiplier, so it can compute this authoritatively where consumers cannot.
2. **`github_copilot.cost.usage`** (unit USD) — direct parity with `claude_code.cost.usage`, for accounts on API/consumption pricing.
3. At minimum, a **`multiplier`** attribute on an existing per-model metric, plus a documented "1 premium request per prompt" counter, so consumers can compute cost correctly.

### Example prompts or workflows

In Grafana/Mimir, a dashboard panel would become:

```promql
sum by (gen_ai_response_model) (github_copilot_premium_request_count)
```

instead of today's brittle client-side approximation:

```promql
sum(gen_ai_client_token_usage_sum{gen_ai_token_type="input",gen_ai_response_model=~"claude-opus.*"})*5/1e6
+ sum(gen_ai_client_token_usage_sum{gen_ai_token_type="output",gen_ai_response_model=~"claude-opus.*"})*25/1e6
+ ... # repeated per model class, hard-coded list prices, NOT the real Copilot bill
```

### Additional context

Related, but none cover emitting a cost/credit **OTel metric**:
- #3474 — track dollar amount spent in a session (UI/session, not telemetry)
- #3686 — cost data in Copilot hooks (hooks, not OTel metrics)
- #2471 — OTel telemetry parity with Claude Code (closed; established the metrics that ship today, but no cost metric)

Context: I built a self-hosted Grafana stack for Copilot CLI telemetry and had to fake the cost panel with client-side list-price math, which I can only honestly label "API-equivalent estimate, not your Copilot bill." A CLI-emitted premium-request/cost metric would make this exact and trustworthy.

首先定位 Copilot CLI 现有的 OpenTelemetry exporter，以及 issue 中列出的指标定义。将可用的模型和会话属性与提议的 premium-request 或成本信号进行比较，然后通过 repository 的 telemetry 测试或 validation 路径验证所选指标和 labels。完成的标准是导出一个权威的计费指标，并提供有文档说明的单位以及可用的模型或会话维度。

由索引模型根据 Issue 内容生成。