core: copy the SchemaDescriptor when rebuilding descriptor (grpc#6851)

useMarshalledMessages works by duplicating a ServerServiceDefinition while replacing just the marshallers. It currently does not copy over the SchemaDescriptors, which breaks at least the ProtoReflectionService.

api/src/main/java/io/grpc/ServerInterceptors.java

@@ -197,10 +197,15 @@ public static <T> ServerServiceDefinition useMarshalledMessages(
       wrappedMethods.add(wrapMethod(definition, wrappedMethodDescriptor));
     }
     // Build the new service descriptor
-    final ServerServiceDefinition.Builder serviceBuilder = ServerServiceDefinition
-        .builder(new ServiceDescriptor(serviceDef.getServiceDescriptor().getName(),
-            wrappedDescriptors));
-    // Create the new service definiton.
+    final ServiceDescriptor.Builder serviceDescriptorBuilder =
+        ServiceDescriptor.newBuilder(serviceDef.getServiceDescriptor().getName())
+            .setSchemaDescriptor(serviceDef.getServiceDescriptor().getSchemaDescriptor());
+    for (MethodDescriptor<?, ?> wrappedDescriptor : wrappedDescriptors) {
+      serviceDescriptorBuilder.addMethod(wrappedDescriptor);
+    }
+    // Create the new service definition.
+    final ServerServiceDefinition.Builder serviceBuilder =
+        ServerServiceDefinition.builder(serviceDescriptorBuilder.build());
     for (ServerMethodDefinition<?, ?> definition : wrappedMethods) {
       serviceBuilder.addMethod(definition);
     }