gistfile1.js

var flatSteps = [];
flatMetrics.forEach(function(flatMetric) {
    flatMetric.forEach(function(step) {
        flatSteps.push(step);
	});
});

===========================================================

var flatSteps = flatMetrics.concatMap(function(metric) {
    return metric.steps;
});

Array.prototype.concat = function() {
    var results = [];
	this.forEach(function(subArray) {
		results.push.apply(results, subArray);
	});
	return results;
}

Array.prototype.concatMap = function(projectionFunction) {
    return this.map(function(item) {
        return projectionFunction(item);
    })
    .concat();
}

==========================================
LoDash version :)

_(flatMetrics)
.map(function(metric) {
    return metric.steps;
})
.flatten();