I just pushed this to the emacs-29 branch, as it's a feature within
a new feature for that branch already.
Danny, I also tweaked the implementation slightly to use a hash table
instead of an alist. I didn't actually test this, as I have no server handy
that supplies these $/progress notifications, but it should be functionally
equivalent. let me know if it's not, it's easy to tweak.
Also, I added a missing entry in the eglot.texi manual.
João