fix: always use a UUID for instance ID

We can't use the Heroku instance ID because it can cause conflicting
metrics to be sent with the same instance ID if the app uses clustering.
This is the safest thing to do and it's what EDO are suggesting.

Author: rowanmanning

packages/app-info/README.md

@@ -103,7 +103,7 @@ This is derived from the dyno metadata
 
 ### `appInfo.instanceId`
 
-Get the ID of the instance that's running the application. This is derived from `process.env.HEROKU_DYNO_ID` if present, otherwise it will be set to a random UUID that identifies the currently running process.
+Get the unique identifier for the instance that's running the application. This will be a different UUID for each running process, including cluster workers.
 
 ### `appInfo.semanticConventions`
 

packages/app-info/lib/index.js

@@ -156,7 +156,7 @@ exports.herokuDynoId = process.env.HEROKU_DYNO_ID || null;
  * @readonly
  * @type {string}
  */
-exports.instanceId = process.env.HEROKU_DYNO_ID || randomUUID();
+exports.instanceId = randomUUID();
 
 /**
  * @type {import('@dotcom-reliability-kit/app-info').SemanticConventions}

packages/app-info/test/unit/lib/index.spec.js

@@ -382,27 +382,14 @@ describe('@dotcom-reliability-kit/app-info', () => {
 
 		beforeEach(() => {
 			jest.resetModules();
-			process.env.HEROKU_DYNO_ID = 'mock-heroku-dyno-id';
+			randomUUID = require('node:crypto').randomUUID;
+			randomUUID.mockReturnValue('mock-generated-uuid');
 			appInfo = require('../../../lib');
 		});
 
-		it('is set to `process.env.HEROKU_DYNO_ID`', () => {
-			expect(appInfo.instanceId).toBe('mock-heroku-dyno-id');
-		});
-
-		describe('when `process.env.HEROKU_DYNO_ID` is not defined', () => {
-			beforeEach(() => {
-				jest.resetModules();
-				randomUUID = require('node:crypto').randomUUID;
-				randomUUID.mockReturnValue('mock-generated-uuid');
-				delete process.env.HEROKU_DYNO_ID;
-				appInfo = require('../../../lib');
-			});
-
-			it('is set to a random UUID', () => {
-				expect(randomUUID).toHaveBeenCalledTimes(1);
-				expect(appInfo.instanceId).toBe('mock-generated-uuid');
-			});
+		it('is set to a random UUID', () => {
+			expect(randomUUID).toHaveBeenCalledTimes(1);
+			expect(appInfo.instanceId).toBe('mock-generated-uuid');
 		});
 	});