Get header text from Vaadin 14 Grid (HeaderCell)

I have implemented an export functionality from a Vaadin 7 and Vaadin 8 Grid. Now we migrate to Vaadin 14 and I cannot find a way to get the header text.

I tried to work with the Renderer:

Column column = grid.getColumnByKey(columnKey);
Renderer renderer = column.getRenderer();
// renderer is a ColumnPathRenderer instance but does not provide information about the header.

So I try to get the information from the HeaderRow or better the HeaderCell:

Column column = grid.getColumnByKey(columnKey);
HeaderRow headerRow = grid.getHeaderRows().get(0);
HeaderCell cell = headerRow.getCell(column);
// cell contains a member 'headerRenderer', an instance of TemplateRenderer<SOURCE>, and this
// headerRenderer contains a member 'template' which contains my header text. But there is no
// API to get this information.

So, how can I get the header text from a Vaadin 14 HeaderCell?