Fix escaping for non-string label values

CHANGELOG.md

@@ -9,6 +9,10 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 This release marks our first release under the Prometheus umbrella.
 
+### Fixed
+
+- Fix escaping for non-string label values without regressing string hot path
+
 ### Breaking
 
 - Drop support for Node.js versions 16, 18, 20, 21 and 23

lib/registry.js

@@ -303,11 +303,12 @@ function flattenSharedLabels(labels) {
 	sharedLabelCache.set(labels, flattened);
 	return flattened;
 }
-function escapeLabelValue(str) {
-	if (typeof str !== 'string') {
-		return str;
+function escapeLabelValue(value) {
+	// Fast-path strings; String() only for non-strings (e.g. Symbols)
+	if (typeof value !== 'string') {
+		value = String(value);
 	}
-	return escapeString(str).replace(/"/g, '\\"');
+	return escapeString(value).replace(/"/g, '\\"');
 }
 function escapeString(str) {
 	return str.replace(/\\/g, '\\\\').replace(/\n/g, '\\n');

test/registerTest.js

@@ -340,6 +340,57 @@ describe('Register', () => {
 			expect(escapedResult).toMatch(/\\"/);
 		});
 
+		it('should escape quotes and newlines in non-string label values', async () => {
+			register.registerMetric({
+				async get() {
+					return {
+						name: 'test_metric',
+						type: 'gauge',
+						help: 'A test metric',
+						values: [
+							{
+								value: 1,
+								labels: {
+									x: ['say "hi"'],
+								},
+							},
+							{
+								value: 2,
+								labels: {
+									y: ['a\nb'],
+								},
+							},
+						],
+					};
+				},
+			});
+			const escapedResult = await register.metrics();
+			expect(escapedResult).toContain('x="say \\"hi\\""');
+			expect(escapedResult).toContain('y="a\\nb"');
+		});
+
+		it('should coerce Symbol label values without throwing', async () => {
+			register.registerMetric({
+				async get() {
+					return {
+						name: 'test_metric',
+						type: 'gauge',
+						help: 'A test metric',
+						values: [
+							{
+								value: 1,
+								labels: {
+									sym: Symbol('x'),
+								},
+							},
+						],
+					};
+				},
+			});
+			const escapedResult = await register.metrics();
+			expect(escapedResult).toContain('sym="Symbol(x)"');
+		});
+
 		describe('should output metrics as JSON', () => {
 			it('should output metrics as JSON', async () => {
 				register.registerMetric(getMetric());