support prometheus metrics (#1853)

Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>
Co-authored-by: Byron Hsu <byronhsu1230@gmail.com>

python/sglang/srt/managers/schedule_batch.py

@@ -31,6 +31,7 @@ ScheduleBatch -> ModelWorkerBatch -> ForwardBatch
 
 import dataclasses
 import logging
+import time
 from typing import List, Optional, Tuple, Union
 
 import torch
@@ -254,6 +255,16 @@ class Req:
         # For Qwen2-VL
         self.mrope_position_delta = []  # use mutable object
 
+        # Lifetime traces
+        # time when request is created and added to waitlist
+        self.created_time = None
+        # time when request is added to prefill batch
+        self.queued_time = None
+        # time when request is being processed
+        self.started_time = None
+        # time when request is finished
+        self.finished_time = None
+
     # whether request reached finished condition
     def finished(self) -> bool:
         return self.finished_reason is not None
@@ -1028,6 +1039,9 @@ class ScheduleBatch:
             f"#req={(len(self.reqs))})"
         )
 
+    def mark_reqs_started(self):
+        for req in self.reqs:
+            req.started_time = time.time()
 
 @dataclasses.dataclass
 class ModelWorkerBatch: