Update ADK doc according to issue #1587 - 13

docs/integrations/secret-manager.md

@@ -0,0 +1,71 @@
+# Google Cloud Secret Manager Integration
+
+The ADK provides a built-in client for Google Cloud Secret Manager to simplify
+retrieving secrets, such as API keys or other credentials, in your production
+deployments.
+
+## SecretManagerClient
+
+The `SecretManagerClient` (`google.adk.integrations.secret_manager.secret_client`)
+provides a simplified interface for retrieving secrets from Secret Manager.
+
+### Authentication
+
+You can instantiate the `SecretManagerClient` in a few ways:
+
+*   **Application Default Credentials (ADC):** If you are running on Google
+    Cloud (e.g., Cloud Run, GKE, Compute Engine) and the environment is
+    configured with a service account, the client will automatically use ADC.
+
+    ```python
+    from google.adk.integrations.secret_manager import secret_client
+
+    secret_manager = secret_client.SecretManagerClient()
+    ```
+
+*   **Service Account JSON:** You can provide the contents of a service account
+    JSON keyfile as a string.
+
+    ```python
+    from google.adk.integrations.secret_manager import secret_client
+    import json
+
+    # Assume service_account_dict contains the service account JSON data
+    service_account_json = json.dumps(service_account_dict)
+
+    secret_manager = secret_client.SecretManagerClient(
+        service_account_json=service_account_json
+    )
+    ```
+
+*   **Authentication Token:** You can use an existing Google Cloud
+    authorization token.
+
+    ```python
+    from google.adk.integrations.secret_manager import secret_client
+
+    auth_token = "YOUR_AUTH_TOKEN"  # Replace with your actual token
+    secret_manager = secret_client.SecretManagerClient(auth_token=auth_token)
+    ```
+
+### Retrieving secrets
+
+To retrieve a secret, use the `get_secret()` method, passing the full
+resource name of the secret version.
+
+```python
+from google.adk.integrations.secret_manager import secret_client
+
+# Instantiate the client (e.g., using ADC)
+secret_manager = secret_client.SecretManagerClient()
+
+# Get the latest version of a secret
+resource_name = "projects/my-project/secrets/my-secret/versions/latest"
+secret_value = secret_manager.get_secret(resource_name)
+
+# Now you can use the secret_value with other ADK tools
+print(f"Retrieved secret: {secret_value}")
+```
+
+Replace `my-project` and `my-secret` with your Google Cloud project ID and
+the name of your secret.