[Bugfix][benchmarks] Fix input token calculation for rerank benchmark metrics (#30596)

Signed-off-by: vensen <vensenmu@gmail.com>

vllm/entrypoints/pooling/score/protocol.py

@@ -120,6 +120,7 @@ class RerankResult(BaseModel):
 
 
 class RerankUsage(BaseModel):
+    prompt_tokens: int
     total_tokens: int
 
 

vllm/entrypoints/pooling/score/serving.py

@@ -502,5 +502,7 @@ class ServingScores(OpenAIServing):
             id=request_id,
             model=model_name,
             results=results,
-            usage=RerankUsage(total_tokens=num_prompt_tokens),
+            usage=RerankUsage(
+                total_tokens=num_prompt_tokens, prompt_tokens=num_prompt_tokens
+            ),
         )