The new spec uses existing EchoDuplicate service in
grpc.testing.duplicate, which refers grpc.testing.Empty,
and verifies if the plugin generates this correctly as
Grpc::Testing::Empty.

- Use full_name() instead of name(), otherwise package names
  are omitted.
- Fix a minor bug on RubyTypeOf(), to replace dotted protobuf
  package name by ruby's module names correctly.

Run those json configurations as part of presubmits

Fix a typo in metrics.proto. No code is using double_value yet and hence no other code changes are needed