MonitoredMode::set calls onModeChange on every round start, so the span was
created whether or not the mode moved. A node with a steady mode therefore
emitted one mode_change per round carrying mode_old == mode_new, which a live
sweep confirmed on every round of both instrumented builds. The round span's
own consensus_mode attribute is still written on every call, since that is
where the round learns the mode it is running in.