Add an express example to demostrate ambient targeting usage (#105)

* support targeting context accessor

* add test

* fix lint

* update

* update

* export targeting context

* add comments

* update

* add express example

* update

* update

* fix lint

* update

* update example in README

* update script

* update

Author: zhiyuanliang-ms

examples/express-app/README.md

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+# Examples for Microsoft Feature Management for JavaScript
+
+These examples show how to use the Microsoft Feature Management in an express application.
+
+## Prerequisites
+
+The examples are compatible with [LTS versions of Node.js](https://github.com/nodejs/release#release-schedule).
+
+## Setup & Run
+
+1. Go to `src/feature-management` under the root folder and run:
+
+    ```bash
+    npm run install
+    npm run build
+    ```
+
+1. Go back to `examples/express-app` and install the dependencies using `npm`:
+
+    ```bash
+    npm install
+    ```
+    
+1. Run the examples:
+
+    ```bash
+    node server.mjs
+    ```
+
+1. Visit `http://localhost:3000/Beta` and use `userId` and `groups` query to specify the targeting context (e.g. /Beta?userId=Jeff or /Beta?groups=Admin). 
+
+    - If you are not targeted, you will get the message "Page not found".
+
+    - If you are targeted, you will get the message "Welcome to the Beta page!".
+
+## Targeting
+
+The targeting mechanism uses the `exampleTargetingContextAccessor` to extract the targeting context from the request. This function retrieves the userId and groups from the query parameters of the request.
+
+```javascript
+const exampleTargetingContextAccessor = {
+    getTargetingContext: () => {
+        const req = requestAccessor.getStore();
+        // read user and groups from request query data
+        const { userId, groups } = req.query;
+        // return aa ITargetingContext with the appropriate user info
+        return { userId: userId, groups: groups ? groups.split(",") : [] };
+    }
+};
+```
+
+The `FeatureManager` is configured with this targeting context accessor:
+
+```javascript
+const featureManager = new FeatureManager(
+    featureProvider, 
+    { 
+        targetingContextAccessor: exampleTargetingContextAccessor 
+    }
+);
+```
+
+This allows you to get ambient targeting context while doing feature flag evaluation.
+
+### Request Accessor
+
+The `requestAccessor` is an instance of `AsyncLocalStorage` from the `async_hooks` module. It is used to store the request object in asynchronous local storage, allowing it to be accessed throughout the lifetime of the request. This is particularly useful for accessing request-specific data in asynchronous operations. For more information, please go to https://nodejs.org/api/async_context.html
+
+```javascript
+import { AsyncLocalStorage } from "async_hooks";
+const requestAccessor = new AsyncLocalStorage();
+```
+
+Middleware is used to store the request object in the AsyncLocalStorage:
+
+```javascript
+server.use((req, res, next) => {
+    requestAccessor.run(req, next);
+});
+```

examples/express-app/config.json

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+{
+  "feature_management": {
+    "feature_flags": [
+      {
+        "id": "Beta",
+        "enabled": true,
+        "conditions": {
+          "client_filters": [
+            {
+              "name": "Microsoft.Targeting",
+              "parameters": {
+                "Audience": {
+                  "Users": [
+                    "Jeff"
+                  ],
+                  "Groups": [
+                    {
+                      "Name": "Admin",
+                      "RolloutPercentage": 100
+                    }
+                  ],
+                  "DefaultRolloutPercentage": 40
+                }
+              }
+            }
+          ]
+        }
+      }
+    ]
+  }
+}

examples/express-app/package.json

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+{
+    "scripts": {
+        "start": "node server.mjs"
+    },
+    "dependencies": {
+        "@microsoft/feature-management": "../../src/feature-management",
+        "express": "^4.21.2"
+    }
+}

examples/express-app/server.mjs

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+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+import fs from "fs/promises";
+import { ConfigurationObjectFeatureFlagProvider, FeatureManager } from "@microsoft/feature-management";
+// You can also use Azure App Configuration as the source of feature flags.
+// For more information, please go to quickstart: https://learn.microsoft.com/azure/azure-app-configuration/quickstart-feature-flag-javascript
+
+const config = JSON.parse(await fs.readFile("config.json"));
+const featureProvider = new ConfigurationObjectFeatureFlagProvider(config);
+
+// https://nodejs.org/api/async_context.html
+import { AsyncLocalStorage } from "async_hooks";
+const requestAccessor = new AsyncLocalStorage();
+const exampleTargetingContextAccessor = {
+    getTargetingContext: () => {
+        const req = requestAccessor.getStore();
+        // read user and groups from request query data
+        const { userId, groups } = req.query;
+        // return an ITargetingContext with the appropriate user info
+        return { userId: userId, groups: groups ? groups.split(",") : [] };
+    }
+};
+
+const featureManager = new FeatureManager(
+    featureProvider, 
+    { 
+        targetingContextAccessor: exampleTargetingContextAccessor 
+    }
+);
+
+import express from "express";
+const server = express();
+const PORT = 3000;
+
+// Use a middleware to store the request object in async local storage.
+// The async local storage allows the targeting context accessor to access the current request throughout its lifetime.
+// Middleware 1 (request object is stored in async local storage here and it will be available across the following chained async operations)
+//   Middleware 2
+//     Request Handler (feature flag evaluation happens here)
+server.use((req, res, next) => {
+    requestAccessor.run(req, next);
+});
+
+server.get("/", (req, res) => {
+    res.send("Hello World!");
+});
+
+server.get("/Beta", async (req, res) => {
+    if (await featureManager.isEnabled("Beta")) {
+        res.send("Welcome to the Beta page!");
+    } else {
+        res.status(404).send("Page not found");
+    }
+});
+
+// Start the server
+server.listen(PORT, () => {
+    console.log(`Server is running at http://localhost:${PORT}`);
+});