feat(metric): bring feature parity between decorator and utility function

packages/metrics/src/Metrics.ts

@@ -25,7 +25,7 @@ class Metrics implements MetricsInterface {
   private isSingleMetric: boolean = false;
   private metadata: { [key: string]: string } = {};
   private namespace?: string;
-  private raiseOnEmptyMetrics: boolean = false;
+  private shouldRaiseOnEmptyMetrics: boolean = false;
   private storedMetrics: StoredMetrics = {};
 
   public constructor(options: MetricsOptions = {}) {
@@ -80,9 +80,19 @@ class Metrics implements MetricsInterface {
     this.storedMetrics = {};
   }
 
+  public captureColdStartMetric(): void {
+    this.captureColdStart();
+  }
+
+  public raiseOnEmptyMetrics(): void {
+    this.shouldRaiseOnEmptyMetrics = true;
+  }
+
   public logMetrics(options: DecoratorOptions = {}): HandlerMethodDecorator {
     const { raiseOnEmptyMetrics, defaultDimensions, captureColdStartMetric } = options;
-    this.raiseOnEmptyMetrics = raiseOnEmptyMetrics || false;
+    if (raiseOnEmptyMetrics) {
+      this.raiseOnEmptyMetrics();
+    }
     if (defaultDimensions !== undefined) {
       this.setDefaultDimensions(defaultDimensions);
     }
@@ -114,7 +124,7 @@ class Metrics implements MetricsInterface {
       Name: metricDefinition.name,
       Unit: metricDefinition.unit,
     }));
-    if (metricDefinitions.length === 0 && this.raiseOnEmptyMetrics) {
+    if (metricDefinitions.length === 0 && this.shouldRaiseOnEmptyMetrics) {
       throw new RangeError('The number of metrics recorded must be higher than zero');
     }
 

packages/metrics/tests/unit/Metrics.test.ts

@@ -225,6 +225,25 @@ describe('Class: Metrics', () => {
   });
 
   describe('Feature: Cold Start', () => {
+    test('Cold start metric should only be written out once and flushed automatically', async () => {
+      const metrics = new Metrics({ namespace: 'test' });
+
+      const handler = async (event: any, context: Context) => {
+        // Should generate only one log
+        metrics.captureColdStartMetric();
+      };
+
+      await handler(dummyEvent, dummyContext);
+      await handler(dummyEvent, dummyContext);
+      const loggedData = [JSON.parse(consoleSpy.mock.calls[0][0])];
+
+      expect(console.log).toBeCalledTimes(1);
+      expect(loggedData[0]._aws.CloudWatchMetrics[0].Metrics.length).toBe(1);
+      expect(loggedData[0]._aws.CloudWatchMetrics[0].Metrics[0].Name).toBe('ColdStart');
+      expect(loggedData[0]._aws.CloudWatchMetrics[0].Metrics[0].Unit).toBe('Count');
+      expect(loggedData[0].ColdStart).toBe(1);
+    });
+
     test('Cold start metric should only be written out once', async () => {
       const metrics = new Metrics({ namespace: 'test' });
 
@@ -338,6 +357,23 @@ describe('Class: Metrics', () => {
         expect((<Error>e).message).toBe('The number of metrics recorded must be higher than zero');
       }
     });
+
+    test('Error should be thrown on empty metrics when raiseOnEmptyMetrics() is callse', async () => {
+      expect.assertions(1);
+
+      const metrics = new Metrics({ namespace: 'test' });
+      const handler = async (event: any, context: Context) => {
+        metrics.raiseOnEmptyMetrics();
+        // Logic goes here
+        metrics.purgeStoredMetrics();
+      };
+
+      try {
+        await handler(dummyEvent, dummyContext);
+      } catch (e) {
+        expect((<Error>e).message).toBe('The number of metrics recorded must be higher than zero');
+      }
+    });
   });
 
   describe('Feature: Auto log at limit', () => {