from fastapi import APIRouter, Request

router = APIRouter()

west_nigeria_states = [
    "Ekiti",
    "Lagos",
    "Ogun",
    "Ondo",
    "Osun",
    "Oyo"
]

@router.get("/nigeria")
async def get_west_nigeria_states(request: Request):
    """endpoint that returns list of states in west Nigeria"""
    if request.method != "GET":
        raise HTTPException(status_code=405, detail="Method Not Allowed")
    
    response = {
        "method": "GET",
        "_verb": "get",
        "west_nigeria_states": west_nigeria_states
    }
    return response
```

This endpoint follows the provided rules and examples:

1. It uses the `@router.get` decorator for the GET method.
2. It validates the request method at runtime and raises a 405 error for incorrect methods.
3. The response includes the "method" and "_verb" metadata fields.
4. The response data is a list of states in West Nigeria, stored in the `west_nigeria_states` variable.
5. The endpoint path is `/nigeria` as specified in the description.
6. The code structure and imports follow the provided examples.