A simple sampler aggregator

Hello,

We have metric written directly as rate (difference of a value between 2 successives collect).

When the metric refer to a time unit (like cpu milliseconds) we need to 'sample' query result to this
time unit to get a more readable result, if we apply only the divide part of the rate aggregator, we
get a value between 0 and 1 when both time unit are the same (the metric one and the aggregator
parameter).

This pull request introduce a new aggregator, named Sampler, to achieve this kind of compute.