support for deriving rates from counters

[dpocock]

should be able to sample counters and convert them to rates for Ganglia

Ganglia expects rates rather than raw counter values

[dpocock]

It is possible to set the slope for a metric to 'postive' instead of 'both'. When slope == 'positive', the RRD file is created with the type COUNTER. It should be able to handle rollover in rrdtool. However, it is not clear whether the rollover handling is only for 32 bit and 64 bit cases or for all the types of value that could come from arbitrary MBeans.