[ceilometer] How to install older version of gnocchi

I use Ubuntu 18.04 to install https://docs.openstack.org/ceilometer/stein/install/install-base-ubuntu.html (ceilometer) When I run service gnocchi-api restart, I get Failed to restart gnocchi-api.service: Unit gnocchi-api.service not found.

I can't find any solution to address this issue. Someone said the new version doesn't include the gnocchi-api, so I am trying to install the previous version, but I don't know where I can find the old version of gnocchi-api https://stackoverflow.com/questions/47520779/service-gnocchi-api-not-found

Can anyone help?

Thanks