Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently Schema::metadata is HashMap<String, String>, whereas Field::metadata is Option<BTreeMap<String, String>>. This is not only inconsistent, but it is unclear why there is an additional Option

Describe the solution you'd like

I would like to change Field::metadata to a HashMap for consistency with Schema

Describe alternatives you've considered

Additional context