perf: improve the memory usage of histogram

.gitignore

@@ -4,3 +4,4 @@ node_modules
 *.log
 coverage/
 package-lock.json
+*.heapsnapshot

CHANGELOG.md

@@ -11,6 +11,8 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- Improve the memory usage of histogram when disable `enableExemplars` option
+
 ### Added
 
 ## [15.1.0] - 2023-12-15

example/histogram-large-memory.js

@@ -0,0 +1,33 @@
+'use strict';
+
+const v8 = require('v8');
+const { register, Histogram } = require('..');
+
+// Create more metrics
+
+async function run() {
+	const labelNames = Array(10).map((_, idx) => `label_${idx}`);
+
+	const getRandomLabels = () =>
+		labelNames.reduce((acc, label) => {
+			return { ...acc, [label]: `value_${Math.random()}` };
+		}, {});
+
+	for (let i = 0; i < 100000; i++) {
+		const h = new Histogram({
+			name: `test_histogram_${i}`,
+			help: `Example of a histogram ${i}`,
+			buckets: [0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1],
+			labelNames,
+		});
+		h.observe(getRandomLabels(), Math.random());
+	}
+
+	await register.metrics();
+
+	global.gc();
+
+	v8.writeHeapSnapshot();
+}
+
+run();

lib/histogram.js

@@ -26,13 +26,6 @@ class Histogram extends Metric {
 		this.defaultExemplarLabelSet = {};
 		this.enableExemplars = false;
 
-		if (config.enableExemplars) {
-			this.enableExemplars = true;
-			this.observe = this.observeWithExemplar;
-		} else {
-			this.observe = this.observeWithoutExemplar;
-		}
-
 		for (const label of this.labelNames) {
 			if (label === 'le') {
 				throw new Error('le is a reserved label keyword');
@@ -45,13 +38,19 @@ class Histogram extends Metric {
 			return acc;
 		}, {});
 
-		this.bucketExemplars = this.upperBounds.reduce((acc, upperBound) => {
-			acc[upperBound] = null;
-			return acc;
-		}, {});
+		if (config.enableExemplars) {
+			this.enableExemplars = true;
+			this.bucketExemplars = this.upperBounds.reduce((acc, upperBound) => {
+				acc[upperBound] = null;
+				return acc;
+			}, {});
+			Object.freeze(this.bucketExemplars);
+			this.observe = this.observeWithExemplar;
+		} else {
+			this.observe = this.observeWithoutExemplar;
+		}
 
 		Object.freeze(this.bucketValues);
-		Object.freeze(this.bucketExemplars);
 		Object.freeze(this.upperBounds);
 
 		if (this.labelNames.length === 0) {
@@ -263,13 +262,16 @@ function observe(labels) {
 }
 
 function createBaseValues(labels, bucketValues, bucketExemplars) {
-	return {
+	const result = {
 		labels,
 		bucketValues: { ...bucketValues },
-		bucketExemplars: { ...bucketExemplars },
 		sum: 0,
 		count: 0,
 	};
+	if (bucketExemplars) {
+		result.bucketExemplars = { ...bucketExemplars };
+	}
+	return result;
 }
 
 function convertLabelsAndValues(labels, value) {
@@ -294,7 +296,9 @@ function extractBucketValuesForExport(histogram) {
 				{ le: upperBound },
 				acc,
 				name,
-				bucketData.bucketExemplars[upperBound],
+				bucketData.bucketExemplars
+					? bucketData.bucketExemplars[upperBound]
+					: null,
 				bucketData.labels,
 			);
 		});
@@ -312,7 +316,7 @@ function addSumAndCountForExport(histogram) {
 				infLabel,
 				d.data.count,
 				`${histogram.name}_bucket`,
-				d.data.bucketExemplars['-1'],
+				d.data.bucketExemplars ? d.data.bucketExemplars['-1'] : null,
 				d.data.labels,
 			),
 			setValuePair(