Function update_boot_blame_stats 

pub async fn update_boot_blame_stats(
    config: Arc<Config>,
    dbus: Arc<OnceCell<Connection>>,
    dbus_timeout: u64,
    machine_stats: Arc<RwLock<MachineStats>>,
    no_fallback: bool,
) -> Result<()>

Update boot blame statistics with the N slowest units at boot.

Takes the shared D-Bus cell rather than a connection: cache hits and the varlink path return without ever connecting, so a bus-less host only pays for D-Bus when it actually collects over D-Bus.