Hi Guenter/All,
it's not the exact requested solution, but it was at least enough to solve our issue, so im copying it below, if it helps to somebody else in similar case.
our issue was that the backend metrics on s08982.* agents have been increasing dramatically and caused historical metrics.
with description provided by Michael Cook i was able to remove (at least) all of the backend metrics (not just the historical ones, but we could accept this limitation) for the affected agents.
here is the guide for it:
mkdir ../datanew
mkdir ../datanew1
mkdir ../datanew2
mkdir ../datanew3
mkdir ../datanew4
./SmartStorTools.sh keep_agents -agents "SuperDomain\\|s08982.*" -src ../data -dest ../datanew
./SmartStorTools.sh prune -src ../datanew -dest ../datanew1
./SmartStorTools.sh test_regex -metrics "Backends.*" -src ../datanew1 > metrics3.out
./SmartStorTools.sh remove_metrics -metrics "Backends.*" -src ../datanew1 -dest ../datanew4/
./SmartStorTools.sh remove_agents -agents "SuperDomain\\|s08982.*" -src ../data -dest ../datanew2
./SmartStorTools.sh prune -src ../datanew2 -dest ../datanew3
./SmartStorTools.sh merge ../datanew3 ../datanew4
rm -arv ../data
mv ../datanew4/ ../data
thank you (Michael and Guenter) for all your help,
stefan