Function refresh_observation_gauges 

pub fn refresh_observation_gauges(
    registry: &UpstreamRegistry,
    obs: &Observability,
)

Refresh the state-derived observation gauges (breaker state, endpoint health, fail-open picks) from registry into obs (DW-021).

Called at scrape time: /metrics reflects a point-in-time snapshot, and the hot paths (pick, report) stay pure atomics with zero metrics coupling. The walk lives here — next to the registry state it reads — and the observability side only exposes plain setters, so metrics depend on nothing. Series for endpoints/upstreams removed by a reload linger until process restart (documented Prometheus caveat).