Add Express example (#577)

* Add Express example

* fix review comments

Author: mayurkale22

examples/express/README.md

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+# Overview
+
+OpenCensus HTTP Instrumentation allows the user to automatically collect trace data and export them to the backend of choice (we are using Zipkin for this example), to give observability to distributed systems.
+
+> NOTE: Please ensure that you start the OpenCensus tracer BEFORE initializing your express app, if you want to enable automatic tracing for built-in plugins (HTTP in this case). https://github.com/census-instrumentation/opencensus-node#plugins
+
+## Installation
+
+```sh
+$ # from this directory
+$ npm install
+```
+
+Setup [Zipkin Tracing](https://opencensus.io/codelabs/zipkin/#0)
+
+## Run the Application
+
+ - Run the server
+
+   ```sh
+   $ # from this directory
+   $ npm start
+   ```
+
+ - Execute the request
+
+    Please visit http://localhost:3000/users to submit the request.
+
+ - Viewing traces
+
+    On navigating to the Zipkin UI at http://localhost:9411
+
+    ![Screenshot](./zipkin.png)
+
+## Useful links
+- For more information on OpenCensus, visit: <https://opencensus.io/>
+- To checkout the OpenCensus for Node.js, visit: <https://github.com/census-instrumentation/opencensus-node>
+- For more information on Zipkin, visit https://zipkin.io/
+
+## LICENSE
+
+Apache License 2.0

examples/express/index.js

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+/**
+ * Copyright 2019, OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+/**
+ * The trace instance needs to be initialized first, if you want to enable
+ * automatic tracing for built-in plugins (HTTP in this case).
+ * https://github.com/census-instrumentation/opencensus-node#plugins
+ */
+
+const port = process.env.APP_PORT || 3000;
+const path = require('path');
+const cookieParser = require('cookie-parser');
+const tracing = require('@opencensus/nodejs');
+const propagation = require('@opencensus/propagation-b3');
+
+// Creates Zipkin exporter
+const zipkin = require('@opencensus/exporter-zipkin');
+const exporter = new zipkin.ZipkinTraceExporter({
+  url: 'http://localhost:9411/api/v2/spans',
+  serviceName: 'opencensus-express'
+});
+
+// NOTE: Please ensure that you start the tracer BEFORE initializing express app
+// Starts tracing and set sampling rate, exporter and propagation
+tracing.start({
+  exporter,
+  samplingRate: 1, // For demo purposes, always sample
+  propagation: new propagation.B3Format(),
+  logLevel: 1 // show errors, if any
+});
+
+const express = require('express');
+const rp = require('request-promise');
+const app = express();
+app.use(express.json());
+app.use(express.urlencoded({ extended: false }));
+app.use(cookieParser());
+app.use(express.static(path.join(__dirname, 'public')));
+
+app.get('/users', function (req, res) {
+  console.log(`user headers: ${JSON.stringify(req.headers)}`);
+  rp('http://localhost:3000/sample').then(data => {
+    res.send(data);
+  }, err => {
+    console.error(`${err.message}`);
+  });
+});
+
+app.get('/sample', function (req, res) {
+  console.log(`sample headers: ${JSON.stringify(req.headers)}`);
+  rp('https://jsonplaceholder.typicode.com/todos/1').then(data => {
+    res.send(data);
+  });
+});
+
+const http = require('http');
+app.set('port', port);
+const server = http.createServer(app);
+server.listen(port);

examples/express/package.json

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+{
+  "name": "express-example",
+  "version": "0.0.1",
+  "description": "Example of Express integration with OpenCensus",
+  "repository": "census-instrumentation/opencensus-node",
+  "keywords": [
+    "opencensus",
+    "http",
+    "tracing",
+    "stats",
+    "metrics"
+  ],
+  "author": "OpenCensus Authors",
+  "license": "Apache-2.0",
+  "engines": {
+    "node": ">=8"
+  },
+  "scripts": {
+    "start": "node index.js",
+    "lint": "semistandard *.js",
+    "fix": "semistandard --fix"
+  },
+  "dependencies": {
+    "@opencensus/exporter-zipkin": "^0.0.14",
+    "@opencensus/nodejs": "^0.0.14",
+    "@opencensus/propagation-b3": "^0.0.14",
+    "cookie-parser": "^1.4.4",
+    "express": "^4.17.0",
+    "http": "*",
+    "method-override": "^3.0.0",
+    "request": "^2.88.0",
+    "request-promise": "^4.2.4"
+  },
+  "devDependencies": {
+    "semistandard": "^13.0.1"
+  }
+}