Allow alert access to previous values

For advanced alerts, we would like to have access to the previous values of errors, discards, etc. This would allow us to generate percentage based values for alerts. For instance, we could generate a much more relevant value for interface discards if we could access the current and last values for discards and UcastPkts, a formula such as "delta(ifInDiscards) / delta(ifHCInUcastPkts)" would let us track the percentage of inbound packet being discarded. Thank you.