Do not initialize route metrics in import_control hook
During route export, the receiving protocol often initialized route metrics to default value in its import_control hook before export filter was executed. This is inconsistent with the expectation that an export filter would process the same route as one in the routing table and it breaks setting these metrics before (e.g. for static routes directly in static protocol). The patch removes the initialization of route metrics in import_control hook, the default values are already handled in rt_notify hook called after export filters. The patch also changed the behavior of OSPF to keep metrics when a route is reannounced between OSPF instances (to be consistent with other protocols) and the behavior when both ospf_metric1 and ospf_metric2 are specified (to have more expected behavior).