Fix prefill delayer wait histograms always observing 0 (#25975)

Co-authored-by: kingjameschan <170807154+kingjameschan@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Humphrey <181440142+humphreysun98@users.noreply.github.com>

test/registered/scheduler/test_prefill_delayer.py

@@ -66,6 +66,9 @@ class NegotiateTestCase:
     # to exercise the legacy slot-only code paths.
     queue_min_ratio: Optional[float] = None
     max_delay_ms: Optional[float] = None
+    # Expected accumulated wait surfaced on the final (release) outcome. When
+    # set, asserts the wait histograms would observe this value instead of 0.
+    expected_wait_forward_passes: Optional[int] = None
 
 
 def _run_negotiate_test(rank, test_cases):
@@ -113,6 +116,17 @@ def _run_negotiate_test(rank, test_cases):
             case.expected_reason,
         ), f"Case {case.name} rank {rank}"
 
+        if case.expected_wait_forward_passes is not None:
+            assert result.wait_forward_passes == case.expected_wait_forward_passes, (
+                f"Case {case.name} rank {rank}: wait_forward_passes "
+                f"{result.wait_forward_passes} != {case.expected_wait_forward_passes}"
+            )
+            # On a release after a real wait, seconds must be observed too.
+            if case.expected_wait_forward_passes > 0:
+                assert (
+                    result.wait_seconds > 0.0
+                ), f"Case {case.name} rank {rank}: wait_seconds not surfaced"
+
 
 _NEGOTIATE_TEST_CASES = [
     NegotiateTestCase(
@@ -127,6 +141,8 @@ _NEGOTIATE_TEST_CASES = [
         ],
         expected_allow=True,
         expected_reason="no_wait",
+        # No prior wait, so the histograms legitimately observe 0.
+        expected_wait_forward_passes=0,
     ),
     NegotiateTestCase(
         name="all_prefillable_with_previous_wait",
@@ -144,6 +160,9 @@ _NEGOTIATE_TEST_CASES = [
         ],
         expected_allow=True,
         expected_reason="wait_success",
+        # One mixed delay preceded the release, so the wait histograms must
+        # observe 1 forward pass (regression guard for #25949).
+        expected_wait_forward_passes=1,
     ),
     NegotiateTestCase(
         name="none_prefillable",
@@ -230,6 +249,9 @@ _NEGOTIATE_TEST_CASES = [
         ],
         expected_allow=True,
         expected_reason="wait_timeout",
+        # Two delays accumulated before timing out; the timeout release must
+        # still surface that wait to the histograms.
+        expected_wait_forward_passes=2,
     ),
     # Queue-based trigger: waiting queue below queue_min = min(running * R,
     # max_prefill_bs) should defer prefill. With R=0.5, running=100 and
@@ -346,6 +368,8 @@ _NEGOTIATE_TEST_CASES = [
         ],
         expected_allow=True,
         expected_reason="wait_success",
+        # One queue-trigger delay was recorded before the wall-clock release.
+        expected_wait_forward_passes=1,
     ),
 ]