feat: Extend API Key Support

.readme-partials.yaml

@@ -220,6 +220,26 @@ body: |-
 
   This method will throw if the token is invalid.
 
+  #### Using an API Key
+
+  An API key can be provided to the constructor:
+  ```js
+  const client = new OAuth2Client({
+    apiKey: 'my-api-key'
+  });
+  ```
+
+  Note, classes that extend from this can utilize this parameter as well, such as `JWT` and `UserRefreshClient`.
+
+  Additionally, an API key it can be used in `GoogleAuth` via the `clientOptions` parameter and will be passed to any generated `OAuth2Client` instances:
+  ```js
+  const auth = new GoogleAuth({
+    clientOptions: {
+      apiKey: 'my-api-key'
+    }
+  })
+  ```
+
   ## JSON Web Tokens
   The Google Developers Console provides a `.json` file that you can use to configure a JWT auth client and authenticate your requests, for example when using a service account.
 

src/auth/oauth2client.ts

@@ -487,6 +487,10 @@ export interface OAuth2ClientEndpoints {
 }
 
 export interface OAuth2ClientOptions extends AuthClientOptions {
+  /**
+   * An API key to use, optional.
+   */
+  apiKey?: string;
   clientId?: string;
   clientSecret?: string;
   redirectUri?: string;
@@ -559,6 +563,7 @@ export class OAuth2Client extends AuthClient {
     this._clientId = opts.clientId;
     this._clientSecret = opts.clientSecret;
     this.redirectUri = opts.redirectUri;
+    this.apiKey = opts.apiKey;
 
     this.endpoints = {
       tokenInfoUrl: 'https://oauth2.googleapis.com/tokeninfo',

test/test.googleauth.ts

@@ -1423,12 +1423,14 @@ describe('googleauth', () => {
     });
 
     it('should pass options to the JWT constructor via constructor', async () => {
+      const apiKey = 'my-api-key';
       const subject = 'science!';
       const auth = new GoogleAuth({
         keyFilename: './test/fixtures/private.json',
-        clientOptions: {subject},
+        clientOptions: {apiKey, subject},
       });
       const client = (await auth.getClient()) as JWT;
+      assert.strictEqual(client.apiKey, apiKey);
       assert.strictEqual(client.subject, subject);
     });
 

test/test.oauth2.ts

@@ -74,6 +74,13 @@ describe('oauth2', () => {
       sandbox.restore();
     });
 
+    it('should accept and set an `apiKey`', () => {
+      const API_KEY = 'TEST_API_KEY';
+      const client = new OAuth2Client({apiKey: API_KEY});
+
+      assert.equal(client.apiKey, API_KEY);
+    });
+
     it('should generate a valid consent page url', done => {
       const opts = {
         access_type: ACCESS_TYPE,