Figure out why there're server descriptors without observed bandwidth
As commented at #40116 (closed), the reason for this might be that sbws is configured to gett "early", "useless" descriptors or it doesn't check the running (and others) flag or relay in dormant mode (#33166 (moved)).
Though this currently doesn't affect sbws reporting (we aren't scaling relays without this data), it'd be nice to know exactly why this happens.