This MR is to react for #549 (closed) and https://gitlab.cern.ch/lhcb-dpa/project/-/issues/243 .
The expected rate of mengzhen_Prescale_SpruceBandQ_BdToEtacKpPim_EtacToHHHH is too high in the centralized test. The rate should be reduced with a factor of a few percents. Prescale it for now. (May tighten the selection and release the prescale in the future.)