fix #283 issue

[odnobit]

openstack v1.6.0 stable:

curl 1.2.3.4:9198/metrics -s | grep "router{"
    ... nothing ...

in logs we can see this error:
time="2023-04-02T09:19:52Z" level=error msg="Failed to collect metric for exporter: neutron, error: failed to collect metric: routers, error: Resource not found: [GET https://1.2.3.4:9696/v2.0/routers/1f60084d-b713-47c7-966a-e3b1f8b442d5/l3-agents], error message: {\"NeutronError\": {\"type\": \"HTTPNotFound\", \"message\": \"The resource could not be found.\", \"detail\": \"\"}}" source="exporter.go:124"

with my fix:

curl 1.2.3.4:9198/metrics -s | grep "router{"
    openstack_neutron_router{admin_state_up="true",external_network_id="",id="1f60084d-b713-47c7-966a-e3b1f8b442d5",name="r",project_id="35f0551d8cf547bca896b09ba1162e70",status="ACTIVE"} 1
		....