`extra` argument to `cli_progress_update()` is either broken or lacks clear usage examples

I've been attempting to set the value of parts of a progress bar using the `extra` argument. When I looked for any documentation above the basic usage of the `extra` argument, there is nothing to go off. None of the examples in the documentation or the vignettes use this functionality at all. As such it's a little hard to debug whether one is simply misunderstanding how variables passed in through `extra` need to be referred to in the format string (`{extra$var1}`? `{var1}`? `{pb$extra$var1}`?), or whether there's actually a deeper bug in the way I'm calling it. As a minimal reprex, the following code throws an error: ``` r cli::cli_progress_bar(total = 10, format = "{testvar}{cli::pb_bar}", extra = list(testvar=1)) for (x in 1:10){ cli::cli_progress_update(extra=list(testvar=x)) Sys.sleep(1) } ``` ``` Error in `"id" %in% names(args)`: ! Could not evaluate cli `{}` expression: `testvar`. Caused by error in `eval(expr, envir = envir)`: ! object 'testvar' not found ``` I assume I'm just misunderstanding how data should be passed to a progress bar, or how it should be referred to within a bar format string. However with no clear usage examples I'm finding it a bit hard to tease apart if it's a bug or a _me_ problem.