feat: Improve gaxios exposure (#1794)

* feat: Improve `gaxios` exposure

* docs: clarify

Author: d-goog

src/auth/authclient.ts

@@ -207,6 +207,26 @@ export abstract class AuthClient
     this.forceRefreshOnFailure = opts.forceRefreshOnFailure ?? false;
   }
 
+  /**
+   * Return the {@link Gaxios `Gaxios`} instance from the {@link AuthClient.transporter}.
+   *
+   * @expiremental
+   */
+  get gaxios(): Gaxios | null {
+    if (this.transporter instanceof Gaxios) {
+      return this.transporter;
+    } else if (this.transporter instanceof DefaultTransporter) {
+      return this.transporter.instance;
+    } else if (
+      'instance' in this.transporter &&
+      this.transporter.instance instanceof Gaxios
+    ) {
+      return this.transporter.instance;
+    }
+
+    return null;
+  }
+
   /**
    * Provides an alternative Gaxios request implementation with auth credentials
    */

src/index.ts

@@ -13,7 +13,10 @@
 // limitations under the License.
 import {GoogleAuth} from './auth/googleauth';
 
+// Export common deps to ensure types/instances are the exact match. Useful
+// for consistently configuring the library across versions.
 export * as gcpMetadata from 'gcp-metadata';
+export * as gaxios from 'gaxios';
 
 export {AuthClient, DEFAULT_UNIVERSE} from './auth/authclient';
 export {Compute, ComputeOptions} from './auth/computeclient';