How to get a partialResultToken from the textDocument/documentSymbol request in vscode

Question

In the lsp specification, the documentSymbolParams extend WorkDoneParams and PartialResultParams. I need to need to get a partialResultToken to send back the results of textDocument/documentSymbols request as a partial result. However vscode does not seem to send one. Is there something specific that I need to do to get it to send a token ? Or is this simply a feature that has yet to be implemented ?

I noticed that the corresponding server capabilities, documentSymbolOptions, extends workDoneProgressOptions which takes a boolean flag to instruct the client that the server supports the feature. I set that flag to true in the response to the initialise request but I still do not receive a token.