Get box metadata client from box

plugins/commands/serve/client/box.rb

@@ -61,7 +61,7 @@ module VagrantPlugins
         # @return [Hash] metadata (from metadata_url)
         def metadata
           res = client.metadata(Empty.new)
-          mapper.map(res.metadata, to: Hash)
+          mapper.map(res.metadata, to: Client::BoxMetadata)
         end
 
         # @return [String] metadata url

plugins/commands/serve/mappers/box.rb

@@ -34,6 +34,20 @@ module VagrantPlugins
           )
         end
       end
+
+      class BoxMetadataFromProto < Mapper
+        def initialize
+          inputs = [].tap do |i|
+            i << Input.new(type: SDK::Args::BoxMetadata)
+            i << Input.new(type: Broker)
+          end
+          super(inputs: inputs, output: Client::BoxMetadata, func: method(:converter))
+        end
+
+        def converter(proto, broker)
+          Client::BoxMetadata.load(proto, broker: broker)
+        end
+      end
     end
   end
 end