Annotate FieldNumber constant emitted for proto extended fields.

Example:

extend MyMessage {
  string new_string = 123;
}

Generates the following C++ constant:

kNewStringFieldNumber = 123;

Clicking on it leads to the generated files instead of the proto file. This change fixes that.

Tested:
  added test case for testing annotations emitted for `extend` fields
PiperOrigin-RevId: 842244563

Author: nbeloglazov

src/google/protobuf/compiler/cpp/extension.cc

@@ -131,7 +131,8 @@ void ExtensionGenerator::GenerateFeatureDefaults(io::Printer* p) const {
 
 void ExtensionGenerator::GenerateDeclaration(io::Printer* p) const {
   auto var = p->WithVars(variables_);
-  auto annotate = p->WithAnnotations({{"name", descriptor_}});
+  auto annotate = p->WithAnnotations(
+      {{"name", descriptor_}, {"constant_name", descriptor_}});
   p->Emit(
       {
           {"constant_qualifier",

src/google/protobuf/compiler/cpp/metadata_test.cc

@@ -572,6 +572,36 @@ TEST_F(CppMetadataTest, AnnotatesLazyMessageSemantics) {
                                    {"mutable_mfield", Annotation::ALIAS},
                                    {"clear_mfield", Annotation::SET}});
 }
+
+constexpr absl::string_view kExtensionFieldTestFile = R"(
+    syntax = "proto2";
+    package foo;
+    message Message {
+      extensions 100 to 199;
+    }
+    extend Message {
+      optional int32 extfield = 100;
+    }
+)";
+
+TEST_F(CppMetadataTest, AnnotatesExtensionSemantics) {
+  FileDescriptorProto file;
+  GeneratedCodeInfo info;
+  std::string pb_h;
+  atu::AddFile("test.proto", kExtensionFieldTestFile);
+  EXPECT_TRUE(CaptureMetadata("test.proto", &file, &pb_h, &info, nullptr,
+                              nullptr, nullptr));
+  EXPECT_EQ("extfield", file.extension(0).name());
+
+  // Check annotations for `extfield`.
+  std::vector<int> field_path{
+      FileDescriptorProto::kExtensionFieldNumber,
+      0,
+  };
+  ExpectAnnotationsForPathContain(info, "test.proto", pb_h, field_path,
+                                  {{"extfield", Annotation::NONE},
+                                   {"kExtfieldFieldNumber", Annotation::NONE}});
+}
 }  // namespace
 }  // namespace cpp
 }  // namespace compiler