[C++][Parquet] `parquet::arrow::FileWriter` does not propagate schema-level metadata when `ArrowWriterProperties::store_schema` is false

### Describe the bug, including details regarding any error messages, version, and platform.

When `store_schema` is true the `FileWriter` first copies any existing metadata before storing the serialized schema:
https://github.com/apache/arrow/blob/8169d6e719453acd0e7ca1b6f784d800cca4f113/cpp/src/parquet/arrow/writer.cc#L537-L542

But when `store_schema` is false, the `FileWriter` just returns an empty metadata, and custom metadata is not copied: https://github.com/apache/arrow/blob/8169d6e719453acd0e7ca1b6f784d800cca4f113/cpp/src/parquet/arrow/writer.cc#L531-L534

Could someone confirm if this is intentional or not? It looks like an oversight to me and I have a patch ready to address it.

### Component(s)

Parquet