Function write_metric_list 

pub fn write_metric_list<W: Write>(w: &mut W) -> Result

Render the metric-list discovery output: a tag legend (sched_class / config_gates / [dead]) followed by a per-metric table whose rows show name | tags | description. Tag legend is keyed off the closed-set vocabulary the registry pin test guards (registry_tag_vocabulary_is_closed), so adding a new allowed class or gate fails the test until both the legend and the closed-set table are updated together.

Splits rendering from I/O so tests can drive the formatter into a String buffer; the public run_metric_list entry point is the print wrapper.