Load shedding under pressure (← go-zero's load.AdaptiveShedder, wired from
RestConf.CpuThreshold).
A request is refused only when the server is hot, its CPU at or over
threshold per-mille or recently so, and full, with more requests in flight
than it has shown it can finish. The algorithm is moonward's Shedder; this
binds it to a Clock and to usage, the CPU meter. Without a meter usage
reads zero and nothing is shed, since no MoonBit backend can read one alone.