Show Created At column for volume backups in v3

The change in [1] erroneously only added the
created_at column for volume v2 API and not
also for the volume v3 API.

[1] https://review.opendev.org/c/openstack/python-openstackclient/+/920003

Change-Id: Iae0b4f21cbc31ae7464a79c8f5e01446ca4ff098

openstackclient/tests/unit/volume/v3/test_volume_backup.py

@@ -333,6 +333,7 @@ class TestBackupList(volume_fakes.TestVolume):
         'Status',
         'Size',
         'Incremental',
+        'Created At',
     )
     columns_long = columns + (
         'Availability Zone',
@@ -350,6 +351,7 @@ class TestBackupList(volume_fakes.TestVolume):
                 b.status,
                 b.size,
                 b.is_incremental,
+                b.created_at,
             )
         )
     data_long = []
@@ -362,6 +364,7 @@ class TestBackupList(volume_fakes.TestVolume):
                 b.status,
                 b.size,
                 b.is_incremental,
+                b.created_at,
                 b.availability_zone,
                 volume_backup.VolumeIdColumn(b.volume_id),
                 b.container,

openstackclient/volume/v3/volume_backup.py

@@ -305,6 +305,7 @@ class ListVolumeBackup(command.Lister):
             'status',
             'size',
             'is_incremental',
+            'created_at',
         )
         column_headers = (
             'ID',
@@ -313,6 +314,7 @@ class ListVolumeBackup(command.Lister):
             'Status',
             'Size',
             'Incremental',
+            'Created At',
         )
         if parsed_args.long:
             columns += ('availability_zone', 'volume_id', 'container')

releasenotes/notes/volume-backup-created-at-list-v3-47400b31be5143bc.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Listing volume backups now shows the created_at column when
+    volume v3 API is used.