Create a CopyHeadingTo method for message descriptors.

This will allow users to export any metadata attached to this descriptor without also constructing every nested descriptor.  This is consistent with FileDescriptor::CopyHeadingTo, which serves a similar purpose at a different scope.

PiperOrigin-RevId: 621995153

src/google/protobuf/descriptor.cc

@@ -2791,7 +2791,7 @@ void FileDescriptor::CopySourceCodeInfoTo(FileDescriptorProto* proto) const {
 }
 
 void Descriptor::CopyTo(DescriptorProto* proto) const {
-  proto->set_name(name());
+  CopyHeadingTo(proto);
 
   for (int i = 0; i < field_count(); i++) {
     field(i)->CopyTo(proto->add_field());
@@ -2811,6 +2811,11 @@ void Descriptor::CopyTo(DescriptorProto* proto) const {
   for (int i = 0; i < extension_count(); i++) {
     extension(i)->CopyTo(proto->add_extension());
   }
+}
+
+void Descriptor::CopyHeadingTo(DescriptorProto* proto) const {
+  proto->set_name(name());
+
   for (int i = 0; i < reserved_range_count(); i++) {
     DescriptorProto::ReservedRange* range = proto->add_reserved_range();
     range->set_start(reserved_range(i)->start);

src/google/protobuf/descriptor.h

@@ -339,6 +339,12 @@ class PROTOBUF_EXPORT Descriptor : private internal::SymbolBase {
   // isn't, the result may be garbage.
   void CopyTo(DescriptorProto* proto) const;
 
+  // Fills in the message-level settings of this message (e.g. name, reserved
+  // fields, message options) to `proto`.  This is essentially all of the
+  // metadata owned exclusively by this descriptor, and not any nested
+  // descriptors.
+  void CopyHeadingTo(DescriptorProto* proto) const;
+
   // Write the contents of this descriptor in a human-readable form. Output
   // will be suitable for re-parsing.
   std::string DebugString() const;