text: Title: [Feature] the stream request returns data without usage.token data
  Body: ### Checklist
  
  - [ ] 1. If the issue you raised is not a feature but a question, please raise a discussion at https://github.com/sgl-project/sglang/discussions/new/choose Otherwise, it will be closed.
  - [ ] 2. Please use English, otherwise it will be closed.
  
  ### Motivation
  
  The cost is calculated through the usage.token returned by the HTTP response. Non-stream requests have a usage.token cost, but the returned usage.token for stream results is empty.
  
  🚀 Is it possible to add the number of tokens when the stream returns data? 🚀
  
  tks.
  
  ```
  data: {"id":"e1b9eecee85b4379a5806f53b6d9ec94","object":"chat.completion.chunk","created":1740117922,"model":"/cephfs/public_model/DeepSeek-R1","choices":[{"index":0,"delta":{"role":null,"content":"。","tool_calls":null},"logprobs":null,"finish_reason":"","matched_stop":null}],"usage":null}
  
  data: {"id":"e1b9eecee85b4379a5806f53b6d9ec94","object":"chat.completion.chunk","created":1740117922,"model":"/cephfs/public_model/DeepSeek-R1","choices":[{"index":0,"message":{"role":"","content":""},"delta":{"content":""},"finish_reason":"stop"}],"usage":{"prompt_tokens":0,"completion_tokens":0,"total_tokens":0,"prompt_tokens_details":{"cached_tokens":0},"prompt_cache_hit_tokens":0,"prompt_cache_miss_tokens":0},"system_fingerprint":"","timings":{"prompt_n":0,"prompt_ms":0,"prompt_per_token_ms":0,"prompt_per_second":0,"predicted_n":0,"predicted_ms":0,"predicted_per_token_ms":0,"predicted_per_second":0}}
  
  data: [DONE]
  ```
  
  
  
  ### Related resources
  
  _No response_