fix: replace undefined label with query name as label

frontend/src/lib/__tests__/getLabelName.test.ts

@@ -0,0 +1,84 @@
+import getLabelName from 'lib/getLabelName';
+
+describe('getLabelName', () => {
+	describe('with a legend template', () => {
+		it('substitutes a single variable that exists on the series', () => {
+			const result = getLabelName(
+				{ 'service.name': 'frontend' },
+				'A',
+				'{{service.name}}',
+			);
+			expect(result).toBe('frontend');
+		});
+
+		it('substitutes a template with surrounding literal text', () => {
+			const result = getLabelName(
+				{ 'service.name': 'frontend' },
+				'A',
+				'rate for {{service.name}}',
+			);
+			expect(result).toBe('rate for frontend');
+		});
+
+		it('substitutes multiple variables when all are present', () => {
+			const result = getLabelName(
+				{ 'service.name': 'frontend', 'http.target': 'GET /api' },
+				'A',
+				'{{service.name}} / {{http.target}}',
+			);
+			expect(result).toBe('frontend / GET /api');
+		});
+
+		it('falls back to query name when a referenced variable is missing', () => {
+			const result = getLabelName(
+				{ 'http.target': 'GET /api' },
+				'F1',
+				'{{service.name}}',
+			);
+			expect(result).toBe('F1');
+		});
+
+		it('falls back to query name even if literal text would still render', () => {
+			const result = getLabelName(
+				{ 'http.target': 'GET /api' },
+				'F1',
+				'label = {{label}}',
+			);
+			expect(result).toBe('F1');
+		});
+
+		it('falls back to query name when any of multiple variables is missing', () => {
+			const result = getLabelName(
+				{ 'service.name': 'frontend' },
+				'F1',
+				'{{service.name}} / {{http.target}}',
+			);
+			expect(result).toBe('F1');
+		});
+
+		it('treats a null label value as missing', () => {
+			const result = getLabelName(
+				{ 'service.name': null } as unknown as Record<string, string>,
+				'F1',
+				'{{service.name}}',
+			);
+			expect(result).toBe('F1');
+		});
+	});
+
+	describe('without a legend template', () => {
+		it('returns key="value" pairs for plain labels', () => {
+			const result = getLabelName(
+				{ 'service.name': 'frontend', 'http.target': 'GET /api' },
+				'A',
+				'',
+			);
+			expect(result).toBe('{service.name="frontend",http.target="GET /api"}');
+		});
+
+		it('returns query name when labels are empty', () => {
+			const result = getLabelName({}, 'A', '');
+			expect(result).toBe('A');
+		});
+	});
+});

frontend/src/lib/getLabelName.ts

@@ -18,6 +18,17 @@ const getLabelName = (
 
 		const results = variables.map((variable) => metric[variable]);
 
+		// Fall back to query name if any `{{var}}` references a label that
+		// isn't on this series — avoids rendering "undefined" in the legend.
+		const hasMissingVariable = variables.some(
+			(variable, index) =>
+				legends.includes(`{{${variable}}}`) &&
+				(results[index] === undefined || results[index] === null),
+		);
+		if (hasMissingVariable) {
+			return query;
+		}
+
 		let endResult = legends;
 
 		variables.forEach((e, index) => {