[Bugfix]: fix missing queue_time_start for requests from grammar_queue

**Motivation**
Fixes a bug where requests appended to grammar_queue (typically the first
tool call request requiring grammar resolution) do not have queue_time_start set

**Changes**
- Sets queue_time_start for any incoming request if `add_to_grammar_queue`

Author: CatherineSue

python/sglang/srt/managers/scheduler.py

@@ -899,6 +899,7 @@ def handle_generate_request(
                 add_to_grammar_queue = True
 
         if add_to_grammar_queue:
+            req.queue_time_start = time.time()
             self.grammar_queue.append(req)
         else:
             self._add_request_to_queue(req)