GH-44714: [C++] Keep field metadata for keys and values when importin…

…g a map type via the C data interface (#44715)

### Rationale for this change

Import of a map type from the C data interface drops field metadata (including extension type information) which does not happen when importing a map type from IPC or a list of structs. This affects the ability to roundtrip data through pyarrow/Arrow C++ if extension types are not registered.

### What changes are included in this PR?

The mechanism to import the map type was changed to align with the method used for IPC import.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

The current behaviour was surprising/inconsistent, so I think this PR brings it in more line with the current expectation/documentation.
* GitHub Issue: #44714

Authored-by: Dewey Dunnington <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

cpp/src/arrow/c/bridge.cc

@@ -1310,13 +1310,13 @@ struct SchemaImporter {
     }
 
     bool keys_sorted = (c_struct_->flags & ARROW_FLAG_MAP_KEYS_SORTED);
-    bool values_nullable = value_type->field(1)->nullable();
+
     // Some implementations of Arrow (such as Rust) use a non-standard field name
     // for key ("keys") and value ("values") fields. For simplicity, we override
     // them on import.
-    auto values_field =
-        ::arrow::field("value", value_type->field(1)->type(), values_nullable);
-    type_ = map(value_type->field(0)->type(), values_field, keys_sorted);
+    type_ =
+        std::make_shared<MapType>(value_type->field(0)->WithName("key"),
+                                  value_type->field(1)->WithName("value"), keys_sorted);
     return Status::OK();
   }
 

cpp/src/arrow/c/bridge_test.cc

@@ -3769,6 +3769,10 @@ TEST_F(TestSchemaRoundtrip, RegisteredExtension) {
 TEST_F(TestSchemaRoundtrip, Map) {
   TestWithTypeFactory([&]() { return map(utf8(), int32()); });
   TestWithTypeFactory([&]() { return map(utf8(), field("value", int32(), false)); });
+  TestWithTypeFactory([&]() {
+    return map(utf8(), field("value", int32(), false,
+                             KeyValueMetadata::Make({"meta key"}, {"meta value"})));
+  });
   // Field names are brought in line with the spec on import.
   TestWithTypeFactory(
       [&]() {