cube: Send Go expvars to Cube for Visualization

Pretty documentation at http://godoc.org/github.com/sburnett/cube

Tip for visualization with Cubism

Because expvars are usually monotonic counters, it's often useful to plot their change over the past minute. If you're using Cubism to plot your data, you can do this by subtracting the previous minute's counter:

function metricPerSecond(metric) {
    var current = cube.metric(metric),
        shift = current.shift(-60 * 1000);
    return current.subtract(shift).divide(60);
}

// ... Set up your graphs ...

// For example, to graph the number of mallocs in the past minute.
var allocsPerMinute = context.horizon().metric(metricPerSecond("max(myevents(memstats.Mallocs))"))
                                       .title("memstats.MallocsPerSecond");