Add rate limiting (#1142)

Fixes #1123 

## What is this PR doing?

It adds rate limiting to the logger API.

## What problem is it solving?

Increases protection from spam requests.

## How is the problem addressed?

By tracking the number of requests in `$_SESSION` and returning early if
the limit was reached.

## Testing Instructions

- Checkout this branch
- Copy the `logger.php` file to a PHP local server or start a Docker
server
```
cd packages/playground/website/public/
docker run -d -p 8787:80 --name playground-website -v "$PWD":/var/www/html php:8.0-apache
```
- Open the [API endpoint link](http://127.0.0.1:8787/logger.php)
- Reload 5 times and confirm that the error message changed to _Too many
requests_

**Known limitations**
Because this is implemented using `$_SESSION` it can easily be worked
around using `curl` or by removing session data.
To improve rate limiting we would need support for storing request
counts by IP on the server and there is currently no storage support
except for files.

packages/playground/website/public/logger.php

@@ -4,6 +4,22 @@
 
 header('Content-Type: application/json');
 
+/**
+ * Rate limit logger requests
+ */
+$max_requests_per_hour = 5;
+
+session_start();
+$rate_limit_key = md5('logger_requests_' . date('Y-m-d H'));
+if (!isset($_SESSION[$rate_limit_key])) {
+    $_SESSION[$rate_limit_key] = 0;
+}
+$_SESSION[$rate_limit_key]++;
+
+if ($_SESSION[$rate_limit_key] > $max_requests_per_hour) {
+    response(false, 'Too many requests');
+}
+
 $channel = getenv('SLACK_CHANNEL');
 $token = getenv('SLACK_TOKEN');