fix(counter): use counter total as exemplar value instead of increment

CHANGELOG.md

@@ -14,6 +14,8 @@ This release marks our first release under the Prometheus umbrella.
 - Drop support for Node.js versions 16, 18, 20, 21 and 23
 - Metric internal storage ('hashMap') changed to a separate object, LabelMap. If you have
   subclassed the built-in metric types you may need to adjust your code.
+- Counter exemplar values now record the counter's total value instead of the increment
+  amount, matching the behavior of other Prometheus clients (Fixes [#621](https://github.com/prometheus/client_js/issues/621))
 
 ### Changed
 

lib/counter.js

@@ -82,7 +82,8 @@ class Counter extends Metric {
 		exemplarLabels = this.defaultExemplarLabelSet,
 	} = {}) {
 		this.incWithoutExemplar(labels, value);
-		this.updateExemplar(labels, exemplarLabels, value);
+		const entry = this.store.entry(labels);
+		this.updateExemplar(labels, exemplarLabels, entry.value);
 	}
 
 	updateExemplar(labels, exemplarLabels, value) {

test/exemplarsTest.js

@@ -60,6 +60,31 @@ describe('Exemplars', () => {
 				);
 			});
 
+			it('should use the counter total as the exemplar value, not the increment', async () => {
+				const localRegistry = new Registry();
+				localRegistry.setContentType(regType);
+				const counterInstance = new Counter({
+					name: 'counter_exemplar_total_test',
+					help: 'help',
+					labelNames: ['method', 'code'],
+					enableExemplars: true,
+					registers: [localRegistry],
+				});
+				counterInstance.inc({
+					labels: { method: 'get', code: '200' },
+					exemplarLabels: { traceId: 'trace_id_1' },
+				});
+				counterInstance.inc({
+					value: 3,
+					labels: { method: 'get', code: '200' },
+					exemplarLabels: { traceId: 'trace_id_2' },
+				});
+				const vals = await counterInstance.get();
+				expect(vals.values[0].value).toEqual(4);
+				expect(vals.values[0].exemplar.value).toEqual(4);
+				expect(vals.values[0].exemplar.labelSet.traceId).toEqual('trace_id_2');
+			});
+
 			it('should make histogram with exemplars on multiple buckets', async () => {
 				const histogramInstance = new Histogram({
 					name: 'histogram_exemplar_test',