Error in UI5Metadata when inheriting a web component

[Psidium]

Describe the bug

When inheriting a UI5 Web Component, the metadata gets wrongly merged into the resulting component. The top-most prototype in the hierarchy will override any shared key of properties, slots and events.

To reproduce

Having this file inheriting the TableRow:

import TableRow from "@ui5/webcomponents/dist/TableRow";

class SingleTableRow extends TableRow {
    static get metadata() {
        return {
            ...TableRow.metadata,
            tag: 'ui5-table-single-row',
            slots: {
                cells: {
                    type: Object,
                    multiple: true,
                    individualSlots: true,
                }
            }
        };
    }
}
SingleRow.define();

This class only redeclares the metadata to allow any div into a table, not just ui5-table-cell.

When trying to add a div in this new object, there is an error, this error is thrown because of the getMetadata() method returns the content of the TableRow's metadata instead of this class',

Expected behavior

The object extending the Component can override any property of the metadata property.

Context

• UI5 Web Components version: 0.11
• OS/Platform: Mac

Affected components

All

[MapTo0]

Unfortunately, our base project is not yet ready for public consumption and will evolve and change a lot in the future. We might somehow hack this but I would not recommend it. I hope that if we can achieve the referenced feature (#455) this can be skipped for now.