Function emit_sessions_stalled 

pub(crate) fn emit_sessions_stalled(cause: PixStallCause, delta: i64)

pub(crate) unlike its siblings, because this gauge has a second producer: the manager’s PixFillControl raises PixStallCause::SurbStarved, which the supervisor cannot see. Routing it here rather than straight at the instrument keeps the metric to one emission point and one cfg, and lets both causes be asserted through the probe below.