fix bug in retention.Sub() not cutting the orig string properly
fix bug in retention.Sub() not cutting the orig string properly

raintank/metrictank,raintank/metrictank,raintank/metrictank,raintank/raintank-metric,replay/metrictank,replay/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/raintank-metric,replay/metrictank,replay/metrictank