New aggregator: histogram

[guillaumeautran]

Carbon relay would benefit from a histogram type aggregator with configured buckets. A histogram aggregator makes it possible to configure buckets to count number of occurrences within a given range in the input data.

For example: given a metric serie: 5, 8, 3, 6, 5, 6, 9, 1 and buckets [0-2[, [2-4[, [4-6[, [6-8[ and [8-10[. The histogram aggregation would output: [0-2[ == 1, [2-4[ == 1, [4-6[ == 2, [6-8[ == 3 and [8-10[ == 1.