SGLang observability server-arg field defaults (enable_metrics and friends)

python/sglang/srt/arg_groups/fields/observability.py on main, 2026-10-04. Verbatim: `enable_metrics: A[bool, "Enable log prometheus metrics."] = False` and `enable_metrics_for_all_schedulers: A[bool, "Enable --enable-metrics-for-all-schedulers when you want schedulers on all TP ranks (not just TP 0) to record request metrics separately. This is especially useful when dp_attention is enabled, as otherwise all metrics appear to come from TP 0."] = False`, plus `enable_mfu_metrics = False`. The enable_metrics default of False is the single fact that makes 'SGLang reports no metrics' a true statement about a default install, and the all-schedulers flag is why a multi-rank series set looks like it comes from TP 0.