decrement(labels, value, timestamp) {
		if (value == null) value = 1;

		const item = this.get(labels);
		return this.set((item ? item.value : 0) - value, labels, timestamp);
	}
Decrement value.

@param {Object} labels
@param {Number?} value
@param {Number?} timestamp
@returns
@memberof GaugeMetric