HISTOGRAM --------- Sample a histogram value. Histograms will produce metrics that describe the distribution of the recorded values, namely the minimum, maximum, average, count and the 75th, 85th, 95th and 99th percentiles. Optionally, specify a list of *tags* to associate with the metric.
function Histogram(key, tags, host) {
    Metric.call(this, key, tags, host);
    this.min = Infinity;
    this.max = -Infinity;
    this.sum = 0;
    this.count = 0;
    this.samples = [];
    this.percentiles = [0.75, 0.85, 0.95, 0.99];
}