Add javascript example for tunnel endpoint (#9080)

Additional configuration for javascript servers.
A simple POST endpoint parsing Sentry browser events and redirect them to Sentry

Author: Deveosys

docs/platforms/javascript/common/troubleshooting/index.mdx

@@ -195,6 +195,37 @@ class HomeController < ApplicationController
 end
 ```
 
+```javascript
+  const SENTRY_HOST = "oXXXXXX.ingest.sentry.io"
+  const SENTRY_PROJECT_IDS = ["123456"]
+  
+  export const postAction = async ({ request }) => {
+      try {
+          const envelope = await request.text();
+          const piece = envelope.split("\n")[0];
+          const header = JSON.parse(piece);
+          const dsn = new URL(header["dsn"]);
+          const project_id = dsn.pathname?.replace("/", "");
+  
+          if (dsn.hostname !== SENTRY_HOST) {
+              throw new Error(`Invalid sentry hostname: ${dsn.hostname}`);
+          }
+  
+          if (!project_id || !SENTRY_PROJECT_IDS.includes(project_id)) {
+              throw new Error(`Invalid sentry project id: ${project_id}`);
+          }
+  
+          const upstream_sentry_url = `https://${SENTRY_HOST}/api/${project_id}/envelope/`;
+          await fetch(upstream_sentry_url, { method: "POST", body: envelope });
+  
+          return json({}, { status: 200 });
+      } catch (e) {
+          console.error("error tunneling to sentry", e);
+          return json({ error: "error tunneling to sentry" }, { status: 500 });
+      }
+  };
+```
+
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