Please check your nova.conf to see if metadata was configured in enabled_apis.
The default value of enabled_apis is "enabled_apis = osapi_compute,metadata", this means that metadata was enabled by default, it will be started by nova-api.
If you remove metadata from enabled_apis, you will be able to start up your metadata api.
I think that this is not a valid case.
Please check your nova.conf to see if metadata was configured in enabled_apis.
The default value of enabled_apis is "enabled_apis = osapi_compute, metadata" , this means that metadata was enabled by default, it will be started by nova-api.
If you remove metadata from enabled_apis, you will be able to start up your metadata api.
Thanks.