fix metal provider flag and env loading, and return error if provider not defined

internal/metal/metal.go

@@ -61,5 +61,9 @@ func New(options ...Option) (Client, error) {
 		client.logger = zap.NewNop()
 	}
 
+	if client.provider == nil {
+		return nil, ErrMetalProviderRequired
+	}
+
 	return client, nil
 }