[BUG]: No `usage` in response when tool calling

When a tool calling response is returned, there is no token usage in the response

* with tool calling

```
  "usage": null
```

* without tool calling

```
  "usage": {
    "prompt_tokens": 21,
    "completion_tokens": 406,
    "total_tokens": 427
  }
```

vllm serve has usage for both with / without tool calling