fix: allow subset for shorter response (GoogleCloudPlatform#9819)

## Description

Fixes test project has so many assets that test was too slow.

Author: engelke

asset/snippets/quickstart_exportassets.py

@@ -18,7 +18,7 @@
 import argparse
 
 
-def export_assets(project_id, dump_file_path):
+def export_assets(project_id, dump_file_path, content_type=None):
     # [START asset_quickstart_export_assets]
     from google.cloud import asset_v1
 
@@ -29,8 +29,16 @@ def export_assets(project_id, dump_file_path):
     parent = "projects/{}".format(project_id)
     output_config = asset_v1.OutputConfig()
     output_config.gcs_destination.uri = dump_file_path
+    request_options = {
+        "parent": parent,
+        "output_config": output_config
+    }
+
+    if content_type is not None:
+        request_options["content_type"] = content_type
+
     response = client.export_assets(
-        request={"parent": parent, "output_config": output_config}
+        request=request_options
     )
     print(response.result())
     # [END asset_quickstart_export_assets]

asset/snippets/quickstart_exportassets_test.py

@@ -66,8 +66,13 @@ def dataset(bigquery_client):
 
 
 def test_export_assets(asset_bucket, dataset, capsys):
+    content_type = asset_v1.ContentType.IAM_POLICY
     dump_file_path = "gs://{}/assets-dump.txt".format(asset_bucket)
-    quickstart_exportassets.export_assets(PROJECT, dump_file_path)
+    quickstart_exportassets.export_assets(
+        PROJECT,
+        dump_file_path,
+        content_type=content_type
+    )
     out, _ = capsys.readouterr()
     assert dump_file_path in out