The format of the data exported needs to change when using storeByGroup, it should include the metric names as well as the values, otherwise the data is meaningless at the other end.

message {
path: "/var/lib/opennms/rrd/snmp/1/ucd-memory.jrb"
owner: "127.0.0.1"
timestamp: 1398767095000
value: 5972.0
value: 470836.0
value: 14716.0
value: 135952.0
value: nan
value: 0.0
value: 476808.0
value: 508904.0
}