Update streaming document

Author: tomek-labuk

app/ai-gateway/streaming.md

@@ -139,6 +139,51 @@ The following is an example `llm/v1/completions` route streaming request:
 
 You should receive each batch of tokens as HTTP chunks, each containing one or many server-sent events.
 
+### Token usage in streaming responses {% new_in 3.13 %}
+
+You can receive token usage statistics in an SSE streaming response. Set the following parameter in the request JSON:
+
+```json
+{
+  "stream_options": {
+    "include_usage": true
+  }
+}
+```
+
+When you set this parameter, the `usage` object appears in the final SSE frame, before the `[DONE]` terminator. This object contains token count statistics for the request.
+
+
+The following example shows how to request and process token usage statistics in a streaming response:
+
+```python
+from openai import OpenAI
+
+client = OpenAI(
+    base_url="http://127.0.0.1:8000/openai",
+    api_key="none"
+)
+
+stream = client.chat.completions.create(
+    model="gpt-4",
+    messages=[{"role": "user", "content": "Tell me the history of Kong Inc."}],
+    stream=True,
+    stream_options={"include_usage": True}
+)
+
+for chunk in stream:
+    if chunk.choices and chunk.choices[0].delta.content:
+        print(chunk.choices[0].delta.content, end="", flush=True)
+    if chunk.usage:
+        print("\nDONE. Usage stats:\n")
+        print(chunk.usage)
+```
+
+{:.info}
+> This feature works with any provider and model when `llm_format` is set to `openai` mode.
+>
+> See the [OpenAI API Documentation](https://platform.openai.com/docs/api-reference/chat/create#chat_create-stream_options) for more information on stream options.
+
 ### Response streaming configuration parameters
 
 In the AI Proxy and AI Proxy Advanced plugin configuration, you can set an optional field `config.response_streaming` to one of three values: