Migrate and update example from examples repo (#494)

* Migrate and update example from examples repo

* Simplify example, fix TraceContext, addSpan

examples/node-tracing/.gitignore

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+node_modules

examples/node-tracing/README.md

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+# node-tracing example
+
+## Overview
+
+This example illustrates a simple comment-wall application using Honeycomb's [Beeline for Node](https://docs.honeycomb.io/getting-data-in/javascript/beeline-nodejs/).
+
+It contains examples of:
+
+- Baseline Beeline usage in an Express app
+- Capture of custom metadata on Beeline-generated events
+- Definition of custom spans to augment traces
+- In-app tracing (across functions within the same service)
+
+## Usage:
+
+Find your Honeycomb API key at https://ui.honeycomb.io/account, then:
+
+## Install and Setup
+
+```bash
+npm run setup # installs packages and local beeline as dependency
+```
+
+## Running the main application
+
+Run our sample `wall` service with:
+
+```bash
+# Will run on port 3000
+$ HONEYCOMB_API_KEY=abc123 npm start
+```
+
+### Interacting with your application
+
+You may either use the web UI [`http://localhost:3000`](http://localhost:3000) to read and write messages:
+
+| ![index](./images/index.png) | ![new message](./images/message.png) |
+| :--------------------------: | :----------------------------------: |
+|    View contents of wall     |      Write new message on wall       |
+
+Or `curl` the contents of your wall directly:
+
+```bash
+# Fetch the contents of your wall
+curl localhost:3000
+```
+
+```bash
+# Write a new message to your wall
+curl localhost:3000 -d "message=i'm #tracing with @honeycombio"
+```

examples/node-tracing/app.js

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+/* eslint-env node */
+"use strict";
+const beeline = require("honeycomb-beeline");
+
+const HONEYCOMB_DATASET = process.env.HONEYCOMB_DATASET || "tracing-example";
+const SERVICE_NAME = process.env.SERVICE_NAME || "wall";
+const HONEYCOMB_API_KEY = process.env.HONEYCOMB_API_KEY || "abc123";
+
+beeline({
+  writeKey: HONEYCOMB_API_KEY,
+  dataset: HONEYCOMB_DATASET,
+  serviceName: SERVICE_NAME,
+  httpTraceParserHook: beeline.w3c.httpTraceParserHook, // in case of mixed services
+  httpTracePropagationHook: beeline.w3c.httpTracePropagationHook, // in case of mixed services
+});
+
+const express = require("express");
+const bodyParser = require("body-parser");
+const app = express();
+const PORT = 3000;
+
+const contents = ["first post"];
+
+app.use(bodyParser.urlencoded({ extended: false, type: "*/*" }));
+
+// = MIDDLEWARE ====================================================
+// Wraps HTTP handlers to output evidence of HTTP calls + trace IDs
+// to STDOUT for debugging.
+// =================================================================
+app.use((req, res, next) => {
+  let traceContext = beeline.honeycomb.marshalTraceContext(beeline.getTraceContext());
+  let { traceId } = beeline.honeycomb.unmarshalTraceContext(traceContext);
+  console.log(
+    "Handling request with:",
+    JSON.stringify({
+      method: req.method,
+      path: req.path,
+      traceId: traceId,
+    })
+  );
+
+  next();
+});
+
+// = HANDLER =======================================================
+// Returns the current contents of our "wall".
+// =================================================================
+app.get("/", (req, res) => {
+  res.send(`${contents.join("<br />\n")}
+    <br /><br /><a href="/message">+ New Post</a>`);
+});
+
+// = HANDLER =======================================================
+// Returns a simple HTML form for posting a new message to our wall.
+// =================================================================
+app.get("/message", (req, res) => {
+  res.send(`<form method="POST" action="/">
+		<input type="text" autofocus name="message" /><input type="submit" />
+	</form>`);
+});
+
+// = HANDLER =======================================================
+// Processes a string from the client and saves the message contents.
+// =================================================================
+app.post("/", async (req, res) => {
+  if (typeof req.body.message !== "string") {
+    beeline.addTraceContext("error", "non-string body");
+    res.status(500).send("not a string body");
+    return;
+  }
+
+  const postSpan = beeline.startSpan({ name: "Posting a message" });
+  let body = req.body.message.trim();
+
+  contents.push(body);
+  beeline.addTraceContext({ message: body });
+  beeline.finishSpan(postSpan);
+
+  res.redirect("/");
+});
+
+app.listen(PORT, () => console.log(`'wall' service listening on port ${PORT}}!`));