[Monitor OpenTelemetry] Update Metric Export Example (#31520)

### Packages impacted by this PR
@azure/monitor-opentelemetry

Updated the metric export example to a sample app that properly exports
metrics to Azure Monitor.

### Checklists
- [x] Added impacted package name to the issue description
- [ ] Does this PR needs any fixes in the SDK Generator?** _(If so,
create an Issue in the
[Autorest/typescript](https://github.com/Azure/autorest.typescript)
repository and link it here)_
- [x] Added a changelog (if necessary)

sdk/monitor/monitor-opentelemetry/samples-dev/customMetric.ts

@@ -5,11 +5,7 @@
  * @summary Demonstrates how to run generate custom metrics that will be sent to Azure Monitor
  */
 
-import {
-  useAzureMonitor,
-  AzureMonitorOpenTelemetryOptions,
-  shutdownAzureMonitor,
-} from "@azure/monitor-opentelemetry";
+import { useAzureMonitor, AzureMonitorOpenTelemetryOptions } from "@azure/monitor-opentelemetry";
 import { metrics } from "@opentelemetry/api";
 
 // Load the .env file if it exists
@@ -25,8 +21,11 @@ const options: AzureMonitorOpenTelemetryOptions = {
 
 useAzureMonitor(options);
 
-export async function main() {
-  // Get Meter and create custom metric
+const express = require("express");
+const app = express();
+const PORT = 8080;
+
+async function metricExport(): Promise<void> {
   const meter = metrics.getMeter("testMeter");
   const customCounter = meter.createCounter("TestCounter");
   customCounter.add(1);
@@ -34,8 +33,15 @@ export async function main() {
   customCounter.add(3);
 }
 
-main().catch(async (error) => {
-  console.error("An error occurred:", error);
-  await shutdownAzureMonitor();
-  process.exit(1);
+async function setupRoutes(): Promise<void> {
+  app.get("/", async (_req: any, res: any) => {
+    await metricExport().then(() => {
+      res.send("Metrics sent to Azure Monitor");
+    });
+  });
+}
+
+setupRoutes().then(() => {
+  app.listen(PORT);
+  console.log(`Listening on http://localhost:${PORT}`);
 });