Fix MeteringLabel model to not clear router's tenant id on deletion

foreign_keys parameter of orm.relationship should point to local columns.
Currently for MeteringLabel it points to Router.tenant_id column which causes
routers tenant_id clearing on label deletion.

Closes-Bug: #1249188
Change-Id: Iccc0daf4f6edd537fd7f9e4b2fc4be094543ca5d

neutron/db/metering/metering_db.py

@@ -52,7 +52,8 @@ class MeteringLabel(model_base.BASEV2, models_v2.HasId, models_v2.HasTenant):
     routers = orm.relationship(
         l3_db.Router,
         primaryjoin="MeteringLabel.tenant_id==Router.tenant_id",
-        foreign_keys='Router.tenant_id')
+        foreign_keys='MeteringLabel.tenant_id',
+        uselist=True)
 
 
 class MeteringDbMixin(metering.MeteringPluginBase,

neutron/tests/unit/services/metering/test_metering_plugin.py

@@ -248,6 +248,18 @@ def test_update_metering_label_rules_rpc_call(self):
                                                         expected_del,
                                                         topic=self.topic)
 
+    def test_delete_metering_label_does_not_clear_router_tenant_id(self):
+        tenant_id = '654f6b9d-0f36-4ae5-bd1b-01616794ca60'
+        with self.metering_label(tenant_id=tenant_id,
+                                 no_delete=True) as metering_label:
+            with self.router(tenant_id=tenant_id, set_context=True) as r:
+                router = self._show('routers', r['router']['id'])
+                self.assertEqual(tenant_id, router['router']['tenant_id'])
+                metering_label_id = metering_label['metering_label']['id']
+                self._delete('metering-labels', metering_label_id, 204)
+                router = self._show('routers', r['router']['id'])
+                self.assertEqual(tenant_id, router['router']['tenant_id'])
+
 
 class TestRouteIntPlugin(l3_agentschedulers_db.L3AgentSchedulerDbMixin,
                          test_l3_plugin.TestL3NatIntPlugin):