[Bugfix] Include encoder prompts len to non-stream api usage response (#8861)

Author: Pernekhan

vllm/entrypoints/openai/serving_chat.py

@@ -726,6 +726,8 @@ async def chat_completion_full_generator(
 
         assert final_res.prompt_token_ids is not None
         num_prompt_tokens = len(final_res.prompt_token_ids)
+        if final_res.encoder_prompt_token_ids is not None:
+            num_prompt_tokens += len(final_res.encoder_prompt_token_ids)
         num_generated_tokens = sum(
             len(output.token_ids) for output in final_res.outputs)
         usage = UsageInfo(