labelsToStr(itemLabels, extraLabels) {
		const labels = Object.assign(
			{},
			this.defaultLabels || {},
			itemLabels || {},
			extraLabels || {}
		);
		const keys = Object.keys(labels);
		if (keys.length == 0) return "";

		return (
			"{" +
			keys
				.map(key => `${this.formatLabelName(key)}="${this.escapeLabelValue(labels[key])}"`)
				.join(",") +
			"}"
		);
	}
Convert labels to Prometheus label string

@param {Object} itemLabels
@param {Object?} extraLabels
@returns {String}
@memberof PrometheusReporter