Add --stream-response-default-include-usage server flag (#16711)

python/sglang/srt/server_args.py

@@ -374,6 +374,7 @@ class ServerArgs:
     pp_max_micro_batch_size: Optional[int] = None
     pp_async_batch_depth: int = 0
     stream_interval: int = 1
+    stream_response_default_include_usage: bool = False
     incremental_streaming_output: bool = False
     enable_streaming_session: bool = False
     random_seed: Optional[int] = None
@@ -4155,6 +4156,12 @@ class ServerArgs:
             action="store_true",
             help="Whether to output as a sequence of disjoint segments.",
         )
+        parser.add_argument(
+            "--stream-response-default-include-usage",
+            action="store_true",
+            help="Include usage in every streaming response "
+            "(even when stream_options is not specified).",
+        )
         parser.add_argument(
             "--stream-output",
             action=DeprecatedStoreTrueAction,