For SLE 15 SP3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:s_liehmann/SLE_15_SP3/home:s_liehmann.repo
zypper refresh
zypper install rrdtool-cached
For SLE 15 SP2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:s_liehmann/SLE_15_SP2/home:s_liehmann.repo
zypper refresh
zypper install rrdtool-cached
For SLE 15 SP1 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:s_liehmann/SLE_15_SP1/home:s_liehmann.repo
zypper refresh
zypper install rrdtool-cached