docs: add example with @socket.io/postgres-adapter

[skip ci]

examples/postgres-adapter-example/README.md

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+# Example with `@socket.io/postgres-adapter`
+
+**Table of contents**
+
+<!-- TOC -->
+  * [How to use](#how-to-use)
+  * [Documentation](#documentation)
+<!-- TOC -->
+
+## How to use
+
+```bash
+# start the postgres server
+$ docker compose up -d
+
+# run the cluster
+$ node cluster.js
+
+# run the client
+$ node client.js
+```
+
+## Documentation
+
+The documentation can be found here: https://socket.io/docs/v4/postgres-adapter/

examples/postgres-adapter-example/client.js

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+import { io } from "socket.io-client";
+
+const CLIENTS_COUNT = 3;
+const PORTS = [3000, 3001, 3002];
+
+for (let i = 0; i < CLIENTS_COUNT; i++) {
+  const socket = io(`ws://localhost:${PORTS[i % 3]}`, {
+    // transports: ["polling"],
+    // transports: ["websocket"],
+  });
+
+  socket.on("connect", () => {
+    console.log(`connected as ${socket.id}`);
+  });
+
+  socket.on("connect_error", () => {
+    console.log(`connect_error`);
+  });
+
+  socket.on("disconnect", (reason) => {
+    console.log(`disconnected due to ${reason}`);
+  });
+
+  socket.on("hello", (socketId, pid) => {
+    console.log(`received "hello" from ${socketId} (process: ${pid})`);
+  });
+
+  setInterval(() => {
+    socket.emit("hello");
+  }, 2000);
+}

examples/postgres-adapter-example/cluster.js

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+import cluster from 'node:cluster';
+
+const SERVERS_COUNT = 3;
+
+cluster.setupPrimary({
+  exec: 'server.js',
+});
+
+for (let i = 0; i < SERVERS_COUNT; i++) {
+  cluster.fork({
+    PORT: 3000 + i
+  });
+}

examples/postgres-adapter-example/compose.yaml

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+services:
+  postgres:
+    image: postgres:14
+    ports:
+      - "5432:5432"
+    environment:
+      POSTGRES_PASSWORD: "changeit"

examples/postgres-adapter-example/package.json

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+{
+  "private": true,
+  "name": "postgres-adapter-example",
+  "version": "0.0.1",
+  "type": "module",
+  "dependencies": {
+    "@socket.io/postgres-adapter": "^0.4.0",
+    "pg": "^8.12.0",
+    "socket.io": "^4.7.5",
+    "socket.io-client": "^4.7.5"
+  }
+}

examples/postgres-adapter-example/server.js

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+import { Server } from "socket.io";
+import { createAdapter } from "@socket.io/postgres-adapter";
+import pg from "pg";
+import process from "node:process";
+
+const PORT = process.env.PORT || 3000;
+
+const pool = new pg.Pool({
+  user: "postgres",
+  host: "localhost",
+  database: "postgres",
+  password: "changeit",
+  port: 5432,
+});
+
+await pool.query(`
+  CREATE TABLE IF NOT EXISTS socket_io_attachments (
+      id          bigserial UNIQUE,
+      created_at  timestamptz DEFAULT NOW(),
+      payload     bytea
+  );
+`);
+
+pool.on("error", (err) => {
+  console.error("Postgres error", err);
+});
+
+const io = new Server({
+  adapter: createAdapter(pool)
+});
+
+io.on("connection", (socket) => {
+  socket.on("hello", () => {
+    // send to anyone except the sender
+    socket.broadcast.emit("hello", socket.id, process.pid);
+  });
+});
+
+io.listen(PORT);
+console.log(`server listening on port ${PORT}`);