no such option oslo_messaging_metrics in group [DEFAULT]

[Arnaud Morin]

I used to use olso.messaging to target some RPC calls on some neutron agents.

It does not work with oslo.messaging >12.8.0
12.8.0 is OK
12.9.0 is KO

Involved commit is: https://github.com/openstack/oslo.messaging/commit/bdbb6d62ee20bfd5ffc59f8772a5a0e60614ba90#

Raises:
oslo_config.cfg.NoSuchOptError: no such option oslo_messaging_metrics in group [DEFAULT]

Some example code:

```python
#!/opt/openstack/neutron/bin/python3

from oslo_config import cfg
from oslo_messaging.rpc import transport
from oslo_messaging.rpc.client import RPCClient
from oslo_messaging.target import Target

conf = cfg.ConfigOpts()
conf([], default_config_files=['/etc/neutron/neutron.conf'])

ts = transport.get_rpc_transport(conf)
target = Target(topic='l3_agent.snat251943.9000.dev.cloud.ovh.net')
client = RPCClient(ts, target, retry=None, timeout=10)
client.call({}, 'oslo_rpc_server_ping')
```

```bash
$ pip freeze | grep oslo
oslo.cache==1.33.3
oslo.concurrency==3.29.1
oslo.config==8.8.0
oslo.context==2.22.1
oslo.db==4.45.0
oslo.i18n==3.23.1
oslo.log==3.42.3
oslo.messaging==12.9.0
oslo.metrics==0.4.0
oslo.middleware==3.37.1
oslo.policy==2.1.1
oslo.privsep==1.32.1
oslo.reports==1.29.2
oslo.rootwrap==5.15.2
oslo.serialization==2.28.2
oslo.service==1.38.0
oslo.upgradecheck==0.2.1
oslo.utils==4.1.0
oslo.versionedobjects==1.35.1
```

[Arnaud Morin (arnaud-morin)]

I was able to mitigate this by using conffixture:

from oslo_messaging.conffixture import ConfFixture

...
ConfFixture(conf)

but that seems overkill to me

[Johannes Kulik (jkulik)]

Using `cfg.CONF` instead of `cfg.ConfigOpts()` and `default_config_dirs` instead of `default_config_files` fixed it for me without the need for `ConfFixture`.

[Takashi Kajinami (kajinamit)]

As suggested above, use the cfg.CONF singleton. The problem you see is caused by the fact that you are creating a different instance and you didn't register all the options needed. If you use the singleton then options should be registered when a relevant modules are loaded.