Fix distributed cardinality aggregation when sent over gRPC

[udoprog]

See: https://github.com/spotify/heroic/blob/master/heroic-dist/src/test/java/com/spotify/heroic/GrpcClusterQueryIT.java#L15

For some reason, the calculated cardinality is incorrect when used over gRPC.

This could be related to grpc having less predictive ordering when recombining the results than the jvm transport. It could also be this, in the combination of the cardinality implementation being a statistical method (with some associated error) and that condition triggers it.