$(iconName) markdown string cannot be rendered as icon in vscode hover

[LSTM-Kirigaya]

In vscode extension development, I always use $(iconName) to render a build-in vscode icon or custom registered icon in forend. For example, screenshot of following displays six icon rendered in hover view.

But in tower lsp, the same code cannot work:

pub fn hover_module_declaration(
    server: &LSPServer,
    token_name: &str,
    language_id: &str
) -> Option<Hover> {

    let markdown = MarkupContent {
        kind: MarkupKind::Markdown,
        value: "$(eye) $(heart) $(mark-github) GitHub".to_string()
    };

    let hover = Hover {
        contents: HoverContents::Markup(markdown),
        range: None
    };

    Some(hover)
}

Hover render result:

As you can see "$(eye) $(heart) $(mark-github) GitHub" is not rendered to icons in vscode. I wonder if $(iconName) rendering is part of lsp or vscode does some postprocessing in forend.