[Frontend] Support custom request_id from request (#9550)

Co-authored-by: Yuhong Guo <yuhong.gyh@antgroup.com>

vllm/entrypoints/openai/protocol.py

@@ -284,6 +284,12 @@ class ChatCompletionRequest(OpenAIBaseModel):
             "The priority of the request (lower means earlier handling; "
             "default: 0). Any priority other than 0 will raise an error "
             "if the served model does not use priority scheduling."))
+    request_id: str = Field(
+        default_factory=lambda: f"{random_uuid()}",
+        description=(
+            "The request_id related to this request. If the caller does "
+            "not set it, a random_uuid will be generated. This id is used "
+            "through out the inference process and return in response."))
 
     # doc: end-chat-completion-extra-params