rgw: Return tenant field in bucket_stats function

Fixes: https://tracker.ceph.com/issues/38800 and https://tracker.ceph.com/issues/36681

(cherry picked from commit 9440776)
Signed-off-by: Volker Theile <vtheile@suse.com>

qa/tasks/radosgw_admin_rest.py

@@ -404,6 +404,7 @@ def task(ctx, config):
 
     assert ret == 200
     assert out['owner'] == user1
+    assert out['tenant'] == ''
     bucket_id = out['id']
 
     # TESTCASE 'bucket-stats4','bucket','stats','new empty bucket','succeeds, expected bucket ID'

src/rgw/rgw_bucket.cc

@@ -1432,6 +1432,7 @@ static int bucket_stats(RGWRados *store, const std::string& tenant_name, std::st
 
   formatter->open_object_section("stats");
   formatter->dump_string("bucket", bucket.name);
+  formatter->dump_string("tenant", bucket.tenant);
   formatter->dump_string("zonegroup", bucket_info.zonegroup);
   formatter->dump_string("placement_rule", bucket_info.placement_rule);
   ::encode_json("explicit_placement", bucket.explicit_placement, formatter);