code: public static ExtensionInfo findExtensionByName(final ExtensionRegistry registry,
        final Descriptor containingType, final String name) {
      final int index = name.lastIndexOf(".");
      final String nameLastPart = index == -1 ? name : name.substring(index + 1);
      return registry.findImmutableExtensionByName(containingType.getFullName() + "." + nameLastPart);
    }
docs: Finds extension by its containing type and the dot separated multi-part name.