feat(metrics): adopted Utility class (#548)

* feat: adopted Utility class

* build: update to commons@0.6.0

docs/core/metrics.md

@@ -61,6 +61,8 @@ For a **complete list** of supported environment variables, refer to [this secti
 
 #### Example using AWS Serverless Application Model (SAM)
 
+The `Metrics` utility is instantiated outside of the Lambda handler. In doing this, the same instance can be used across multiple invocations inside the same execution environment. This allows `Metrics` to be aware of things like whether or not a given invocation had a cold start or not.
+
 === "handler.ts"
 
     ```typescript hl_lines="1 4"

packages/metrics/package.json

@@ -84,7 +84,7 @@
     "url": "https://github.com/awslabs/aws-lambda-powertools-typescript/issues"
   },
   "dependencies": {
-    "@aws-lambda-powertools/commons": "^0.2.0",
+    "@aws-lambda-powertools/commons": "^0.6.0",
     "@types/aws-lambda": "^8.10.72"
   }
 }

packages/metrics/src/Metrics.ts

@@ -1,4 +1,5 @@
 import { Callback, Context } from 'aws-lambda';
+import { Utility } from '@aws-lambda-powertools/commons';
 import { MetricsInterface } from '.';
 import { ConfigServiceInterface, EnvironmentVariablesService } from './config';
 import {
@@ -79,20 +80,21 @@ const DEFAULT_NAMESPACE = 'default_namespace';
  * };
  * ```
  */
-class Metrics implements MetricsInterface {
+class Metrics extends Utility implements MetricsInterface {
   private customConfigService?: ConfigServiceInterface;
   private defaultDimensions: Dimensions = {};
   private dimensions: Dimensions = {};
   private envVarsService?: EnvironmentVariablesService;
   private functionName?: string;
-  private isColdStart: boolean = true;
   private isSingleMetric: boolean = false;
   private metadata: { [key: string]: string } = {};
   private namespace?: string;
   private shouldThrowOnEmptyMetrics: boolean = false;
   private storedMetrics: StoredMetrics = {};
 
   public constructor(options: MetricsOptions = {}) {
+    super();
+
     this.dimensions = {};
     this.setOptions(options);
   }
@@ -172,8 +174,7 @@ class Metrics implements MetricsInterface {
    * ```
    */
   public captureColdStartMetric(): void {
-    if (!this.isColdStart) return;
-    this.isColdStart = false;
+    if (!this.isColdStart()) return;
     const singleMetric = this.singleMetric();
 
     if (this.dimensions.service) {