Add length method to RequestSession

Author: sjmonson

src/guidellm/request/session.py

@@ -13,6 +13,9 @@
 
 
 class RequestSession(ABC, Generic[RequestT, ResponseT]):
+    @abstractmethod
+    def __len__(self) -> int: ...
+
     @abstractmethod
     def get_next_request(self) -> RequestT: ...
 
@@ -36,6 +39,9 @@ def __init__(self, prompts: list[GenerationRequest]) -> None:
         self.prompts = prompts
         self.responses: list[str] = []
 
+    def __len__(self) -> int:
+        return len(self.prompts)
+
     def get_next_request(self) -> GenerationRequest:
         completed_responses = len(self.responses)
         base_request = self.prompts[completed_responses].model_copy()