User-defined aggregate function to sum jsonb values

I have agg function that sums decimals. Business logic omitted for brevity

CREATE OR REPLACE FUNCTION calculate_snapshot_internal_decimal(_this_value DECIMAL,
                                                               _next_value DECIMAL
                                                               OUT _code DECIMAL)
    LANGUAGE plpgsql AS
$func$
BEGIN

    _code = _this_value + _next_value;

END
$func$;

Now I need the same for JSONB where key is string and value is always BigDecimal _this_value:

{"weight": "50", "height":"170", "size": "2"}

_next_value

{"weight": "40", "height":"20"}

aggregated result:

{"weight": "90", "height":"190", "size": "2"}

What could I try?