SGLang utilization gauge - DOCUMENTED BUT PERMANENTLY ZERO ON MAIN

Structurally dead on current main and this is a verified reading of the source, not an inference. metrics_reporter._calculate_utilization reads 'TODO: max_running_requests_under_SLO has no setter - sglang:utilization stuck at 0 (regressed #22713)' and only assigns when max_under_slo is not None and > 0, which it never is. Under --disaggregation-mode prefill the same method assigns -1 instead. So the series exists, is exported, and carries either 0 or -1. DO NOT build an autoscaling or saturation rule on it: on a prefill worker 'utilization above 1' would fire on the sentinel value -1 inverted, and elsewhere it never fires at all.