title: Firestore Security Rule allowing unauthenticated access only to one collection
body: <p>I am trying to establish below scenario with the  help of Firestore rules.</p>
  
  <p>How can I let user access 'products' collection without authentication but other collections with authentication? I have tried putting rules as below, but it doesn't work.</p>
  
  <pre><code>service cloud.firestore {
    match /databases/{database}/documents {
      // All should be able to access products collection
      match /products {
          allow read;
      }
      // All other collection should only be accessed if user is authenticated.
      match /{document=**} {
        allow read, write: if request.auth != null;
      }
    }
  }
  </code></pre>