Add docs for --tunnel

deploy/reference/tunnel.md

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+---
+title: Tunnel
+description: "Learn about Deno Deploy's local tunnel feature, which allows secure access to your local development server from the internet."
+---
+
+Deno Deploy's tunnel feature allows you to securely expose your local
+development server to the internet. This is particularly useful for testing
+webhooks, sharing your work with collaborators, or accessing your local server
+from remote locations.
+
+In addition to providing secure access to your local server, Deno Deploy's
+tunnel can also:
+
+- Pull environment variables in the "Local" context from your Deno Deploy
+  project to your local Deno process.
+- Push Open Telemetry traces, metrics, and logs from your local Deno process to
+  your Deno Deploy app, where you can view them in the Deno Deploy dashboard.
+- Automatically connect to local development databases assigned to your Deno
+  Deploy app.
+
+## Getting started
+
+To start using the tunnel feature, you'll need to have Deno installed on your
+local machine. You can then pass the `--tunnel` flag when running your Deno
+application locally, either with `deno task` or `deno run`. For example:
+
+```sh
+deno run --tunnel -A main.ts
+```
+
+The first time you run this command, you'll be prompted to authenticate with
+Deno Deploy and choose which Deno Deploy app you want to connect the tunnel to.
+Once authenticated, a secure tunnel will be established, and you'll receive a
+public URL that forwards traffic to your local server.
+
+You can also specify `--tunnel` for `deno task` commands defined in your
+`deno.json` or `package.json` file:
+
+```json
+{
+  "tasks": {
+    "dev": "astro dev"
+  }
+}
+```
+
+Then run the task with:
+
+```bash
+deno task --tunnel dev
+```
+
+## Using the tunnel
+
+Once the tunnel is established, any requests made to the public URL will be
+forwarded to your local development server. You can use this URL to test
+webhooks, share your work with others, or access your local server from remote
+locations.
+
+## Stopping the tunnel
+
+To stop the tunnel, simply terminate the Deno process running your application.
+This will close the secure connection and stop forwarding traffic to your local
+server.
+
+## View open tunnels
+
+The "Tunnels" tab of your application dashboard on Deno Deploy shows all active
+tunnels connected to your application. From this tab, you can view details about
+each tunnel, including the public URL, the local address it's forwarding to, and
+the time it was established.
+
+## Environment variables
+
+When using the tunnel feature, the "Local" context environment variables from
+your Deno Deploy application are made available to your local Deno process. This
+allows you to use the same configuration locally as you do in your Deno Deploy
+application.
+
+You can view and manage the environment variables for your Deno Deploy
+application in the "Environment Variables" tab of your application settings. See
+[the docs on adding, editing, and removing environment variables](/deploy/reference/env_vars_and_contexts/#adding%2C-editing-and-removing-environment-variables)
+for more information.
+
+## Viewing traces and logs
+
+When using the tunnel feature, Open Telemetry traces, metrics, and logs from
+your local Deno process are pushed to your Deno Deploy application. You can view
+these traces and logs in the "Observability" tab of your application dashboard
+on Deno Deploy.
+
+You can filter to see only traces and logs from your local process by searching
+for `context:local` in the search bar.