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

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

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")