feat: improve performance of getMetricAsPrometheusString (#542)

Co-authored-by: Dan Shappir <[email protected]>

CHANGELOG.md

@@ -11,6 +11,9 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- Refactor `getMetricAsPrometheusString` method in the `Registry` class to use `Array.prototype.join`
+  instead of loop of string concatenations.
+- Also use `Array.prototype.map`, and object spread instead of an explicit `for` loop
 - changed: updated the sample output in `example/default-metrics.js`
 
 ### Added

lib/registry.js

@@ -27,40 +27,27 @@ class Registry {
 		const name = escapeString(item.name);
 		const help = `# HELP ${name} ${escapeString(item.help)}`;
 		const type = `# TYPE ${name} ${item.type}`;
-		const defaultLabelNames = Object.keys(this._defaultLabels);
-
-		let values = '';
-		for (const val of item.values || []) {
-			val.labels = val.labels || {};
-
-			if (defaultLabelNames.length > 0) {
-				// Make a copy before mutating
-				val.labels = Object.assign({}, val.labels);
-
-				for (const labelName of defaultLabelNames) {
-					val.labels[labelName] =
-						val.labels[labelName] || this._defaultLabels[labelName];
-				}
-			}
-
-			let metricName = val.metricName || item.name;
-
-			const keys = Object.keys(val.labels);
-			const size = keys.length;
-			if (size > 0) {
-				let labels = '';
-				let i = 0;
-				for (; i < size - 1; i++) {
-					labels += `${keys[i]}="${escapeLabelValue(val.labels[keys[i]])}",`;
-				}
-				labels += `${keys[i]}="${escapeLabelValue(val.labels[keys[i]])}"`;
-				metricName += `{${labels}}`;
-			}
+		const defaultLabels =
+			Object.keys(this._defaultLabels).length > 0 ? this._defaultLabels : null;
+
+		const values = [help, type];
+		for (const { metricName = item.name, value, labels = {} } of item.values ||
+			[]) {
+			const labelsWithDefaults = defaultLabels
+				? { ...labels, ...defaultLabels, ...labels }
+				: labels;
+
+			const formattedLabels = Object.entries(labelsWithDefaults).map(
+				([n, v]) => `${n}="${escapeLabelValue(v)}"`,
+			);
+			const labelsString = formattedLabels.length
+				? `{${formattedLabels.join(',')}}`
+				: '';
 
-			values += `${metricName} ${getValueAsString(val.value)}\n`;
+			values.push(`${metricName}${labelsString} ${getValueAsString(value)}`);
 		}
 
-		return `${help}\n${type}\n${values}`.trim();
+		return values.join('\n');
 	}
 
 	async metrics() {