Metrics ain't deleted when corresponding resource is deleted

After resource is deleted its metrics still resist in the DB preventing removing of their archive policy.

Steps to reproduce:
1. Create archive-policy:
gnocchi archive-policy create -b 0 -m min -m max -m mean -m count -d points:3,timespan:900 -d points:5,granularity:60 -d timespan:3600,granularity:180 AutoTestResourcePolicy-32121

2. Create generic resource with custom metrics:
gnocchi resource create -t generic -n 'Metric1:AutoTestResourcePolicy-32121' -n 'Metric2:AutoTestResourcePolicy-32121' f53bb4c4-e53b-4a4d-b5c1-22449088f6d9

3. Assert resource and archive-policy exist
    gnocchi archive-policy list
    gnocchi resource list

4. Delete resource:
    gnocchi resource delete f53bb4c4-e53b-4a4d-b5c1-22449088f6d9

5. Try to delete archive-policy
    gnocchi archive-policy delete AutoTestResourcePolicy-32121
    Archive policy AutoTestResourcePolicy-32121 is still in use (HTTP 400)

In gnocchi DB:
select name,archive_policy_name from metric;
+----------------+------------------------------+
| name | archive_policy_name |
+----------------+------------------------------+
| image.serve | low |
| image.size | low |
| FakeMeter3 | low |
| image | low |
| image.download | low |
| FakeMeter1 | low |
| Metric2 | AutoTestResourcePolicy-32121 |
| Metric1 | AutoTestResourcePolicy-32121 |
| FakeMeter2 | low |
+----------------+------------------------------+

Packages: 1.3.0-5
Client: 2.0.0-1

Regards,
Yurii