startTimer(labels) {
		const start = process.hrtime();
		return endLabels => {
			const delta = process.hrtime(start);
			const value = delta[0] + delta[1] / 1e9;
			this.set(Object.assign({}, labels, endLabels), value);
			return value;
		};
	}
Start a timer
@param {object} labels - Object with labels where key is the label key and value is label value. Can only be one level deep
@returns {function} - Invoke this function to set the duration in seconds since you started the timer.
@example
var done = gauge.startTimer();
makeXHRRequest(function(err, response) {
	done(); //Duration of the request will be saved
});