[Bug] Fix missing TTFT histogram for single-batch requests (#20122)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

python/sglang/srt/managers/tokenizer_manager.py

@@ -134,6 +134,7 @@ class ReqState:
     obj: Union[GenerateReqInput, EmbeddingReqInput]
     time_stats: APIServerReqTimeStats
     last_completion_tokens: int = 1
+    ttft_observed: bool = False
 
     # For streaming output
     last_output_offset: int = 0
@@ -1611,12 +1612,13 @@ class TokenizerManager(TokenizerCommunicatorMixin, TokenizerManagerMultiItemMixi
                 }
 
             state.finished = recv_obj.finished_reasons[i] is not None
-            if state.finished:
+
-                # Ensure first_token_time is set before computing decode_throughput.
+            # Set first_token_time on the first output batch.
-                # Without this, requests that finish on their first output batch
+            # This is the single write point for first_token_time.
-                # would have first_token_time=0.0, producing bogus throughput.
             if state.time_stats.first_token_time == 0.0:
                 state.time_stats.set_first_token_time()
+
+            if state.finished:
                 state.time_stats.trace_ctx.trace_set_root_attrs(
                     self.convert_to_span_attrs(state, recv_obj, i)
                 )
@@ -1927,10 +1929,10 @@ class TokenizerManager(TokenizerCommunicatorMixin, TokenizerManagerMultiItemMixi
             if priority is not None:
                 labels["priority"] = str(priority)
         if (
-            state.time_stats.first_token_time == 0.0
+            not state.ttft_observed
             and self.disaggregation_mode != DisaggregationMode.PREFILL
         ):
-            state.time_stats.set_first_token_time()
+            state.ttft_observed = True
             state.last_completion_tokens = completion_tokens
             self.metrics_collector.observe_time_to_first_token(
                 labels, state.time_stats.get_first_token_latency()

test/registered/metrics/test_priority_metrics.py

@@ -19,7 +19,6 @@ from sglang.test.test_utils import (
 register_cuda_ci(
     est_time=60,
     suite="stage-b-test-small-1-gpu",
-    disabled="Flaky: no TTFT histogram samples. See https://github.com/sgl-project/sglang/actions/runs/22787468608/job/66118130037",
 )
 register_amd_ci(est_time=60, suite="stage-b-test-small-1-gpu-amd")