feat: Add a response header to understand who responded

Adds a new `X-Gu-Target-Group` header to every response to help understand which target group served the request, EC2 or ECS.

Author: akash1810

dotcom-rendering/src/server/server.dev.ts

@@ -1,4 +1,11 @@
-import { type ErrorRequestHandler, type Handler, Router } from 'express';
+import {
+	type ErrorRequestHandler,
+	type Handler,
+	type NextFunction,
+	type Request,
+	type Response,
+	Router,
+} from 'express';
 import { pages } from '../devServer/routers/pages';
 import { targets } from '../devServer/routers/targets';
 import { handleAllEditorialNewslettersPage } from './handler.allEditorialNewslettersPage.web';
@@ -105,6 +112,12 @@ const renderer = Router();
 renderer.use(getContentFromURLMiddleware);
 renderer.use(getABTestsFromQueryParams);
 renderer.use(requestLoggerMiddleware);
+
+renderer.use((req: Request, res: Response, next: NextFunction) => {
+	res.setHeader('X-Gu-Target-Group', 'dev');
+	next();
+});
+
 renderer.get('/Article/*url', handleArticle);
 renderer.get('/Interactive/*url', handleInteractive);
 renderer.get('/Blocks/*url', handleBlocks);

dotcom-rendering/src/server/server.prod.ts

@@ -1,5 +1,10 @@
 import compression from 'compression';
-import type { ErrorRequestHandler, Request, Response } from 'express';
+import type {
+	ErrorRequestHandler,
+	NextFunction,
+	Request,
+	Response,
+} from 'express';
 import express from 'express';
 import { NotRenderableInDCR } from '../lib/errors/not-renderable-in-dcr';
 import { handleAllEditorialNewslettersPage } from './handler.allEditorialNewslettersPage.web';
@@ -41,6 +46,15 @@ export const prodServer = (): void => {
 	app.use(requestLoggerMiddleware);
 	app.use(compression());
 
+	app.use((req: Request, res: Response, next: NextFunction) => {
+		// See https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-environment-variables.html
+		const runningInECS =
+			process.env.AWS_EXECUTION_ENV?.startsWith('AWS_ECS_') === true;
+
+		res.setHeader('X-Gu-Target-Group', runningInECS ? 'ecs' : 'ec2');
+		next();
+	});
+
 	app.get('/_healthcheck', (req: Request, res: Response) => {
 		res.status(200).send('OKAY');
 	});