Implement a runtime requirement checking if API key is not set

I think we then don't need to throw the exception at run time.
Eugene, what do you reckon here ?

1. +++ b/google_vision.install
   @@ -0,0 +1,26 @@
   +    $key = \Drupal::service('config.factory')->get('google_vision.settings')->get('api_key');
   

   use \Drupal::config('google_vision.settings')->get('api_key');

2. +++ b/google_vision.install
   @@ -0,0 +1,26 @@
   +        ':settings' => \Drupal::url('google_vision.settings'),
   

   use Url::fromRoute instead of \Drupal::url

let's do it.
create a separate issue for Handling errors in Response.

+++ b/google_vision.install
@@ -0,0 +1,28 @@
+        'description' => t('The API key is not set at /admin/config/media/google_vision. It is highly required to set the API Key at the <a href=":settings">configuration page</a>.', array(

"The API key is not set at /admin/config/media/google_vision. It is highly required to set the API Key at the :settings page."

I don't see the point of giving an end user a message with a path.
Also, I don't think "highly required" makes sense, it is required or it is not.

Let's change it to something like
"Google Vision API key is not set and it is required for some functionalities to work. Please set it up at the :Google Vision module settings page."

+++ b/google_vision.install
@@ -0,0 +1,28 @@
+        'title' => t('API Key configuration'),

Also, let's include Google Vision API as the title, so we have the right context. See the following screenshot, and you won't know which API key are we talking about here.

+++ b/google_vision.install
@@ -0,0 +1,28 @@
+        'description' => t('Google Vision API key is not set and it is required for some functionalities to work. Please set it up at the <a href=":settings">Google Vision module settings page</a>.', array(

The link is not working.

I think this is a perfect issue for setting up our first WebTest. Please read https://www.drupal.org/contributor-tasks/write-tests for getting started.

This looks really great, congrats. Some minor comments:

1. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,57 @@
   +  /**
   +   * Modules to install.
   +   *
   +   * @var array
   +   */
   

   Use {@inheritdoc}

2. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,57 @@
   +    $config = $this->container->get('config.factory')->get('google_vision.settings');
   

   Is $config used there? I think that statement is useless.

3. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,57 @@
   +    if($this->assertFieldByName('api_key', '')) {
   

   Don't use an if. If the field doesn't exist, it will fail the test already.

4. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,57 @@
   +      $this->assertText('Google Vision API key is not set and it is required for some functionalities to work.', 'The content exists on the report page');
   

   And assert that the link goes to the right page.

+++ b/src/Tests/ApiKeyConfigurationTest.php
@@ -0,0 +1,56 @@
+   * Tests that the Google Vision API Key is set in the admin config page.

I forgot: this phpdoc is not accurate, that's not what we are testing.

But we should add a test for testing that too.

We still need a test for saving the Google Vision API Key and verify that it's stored successfully.

+++ b/src/Tests/ApiKeyConfigurationTest.php
@@ -21,6 +21,15 @@ class ApiKeyConfigurationTest extends WebTestBase {
+   * Disable strict config schema checking.
+   *
+   * The schema is verified at the end of running the update.
+   *
+   * @var bool
+   */
+  protected $strictConfigSchema = FALSE;

Why we are restricting strictconfigschema ?
Define the schema/metadata of the config being used.
See https://www.drupal.org/node/1905070

1. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,73 @@
   +use Drupal\Core\Config\ConfigFactoryInterface;
   

   This use statement is not needed I think? Just looking at the patch, not in context.

2. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,73 @@
   +  /**
   +   * Modules to install.
   +   *
   +   * @var array
   +   */
   +  public static $modules = ['google_vision'];
   

   This should be {@inheritdoc}

Otherwise looks good to me.

1. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,73 @@
   +    $this->adminUser = $this->drupalCreateUser(array('administer google vision','administer site configuration')
   +    );
   

   This new line is weird too. And I would prefer short array syntax, but that's a matter of taste I guess.

2. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,73 @@
   +
   

   Extra empty line.

3. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,73 @@
   +    $apiKey = $this->randomString(40);
   

   Variables are usually using underscore vs camelCase. See https://www.drupal.org/coding-standards#naming

4. +++ b/src/Tests/ApiKeyConfigurationTest.php
   @@ -0,0 +1,73 @@
   +    $data = array(
   +      'api_key' => $apiKey,
   +    );
   

   Usually we use $edit as a name for this variables, but I don't care that much. Maybe you want to change it for consistence.
   Also, can be a single line.

+++ b/src/Tests/ApiKeyConfigurationTest.php
@@ -0,0 +1,68 @@
+  function testKeyConfiguration() {

Add the access modifier of the function. same as below one.

Looks good to me, thanks :D

Patch looks okay, but in this patch you're modifying the file google_vision.schema.yml. This file is not exists yet. Maybe another issue should be solved befor it?

The patch needs reroll as the permission of the google_vision.schema.yml was changed from 755 to 644 in #2731801: Use of services to access functions
Here's the updated patch with the permission fixing.Hope this will cleanly apply

@ajalan065,
There's nothing major needs to be done here.

let's go for it and enable to drupalci on project :)

Adding proposed commit mesage

@ajalan
sorry, I was wrong about the file config/schema/google_vision.schema.yml

Thanks! There's was not my code so doing RTBC it.

Committed and pushed to 8.x-1.x
Well now we have tests :)

updating commit message.