Add fontification for progress cookie in org agenda

In Org mode, there is an easy way to visualize progress on a task with subtasks. You can place a "progress cookie" [/] or [%] in the title of a TODO task or parent item in a list, hit C-c C-c and Org will calculate the progress for you.

* TODO foo [1/3]
** TODO one
** TODO two
** DONE three
- parent [66%]
  - [X] one
  - [X] two
  - [ ] three

These are fontified with org-checkbox-statistics-todo to make them easily stand out. However, for some reason this face is not applied in the Org agenda buffer.

Because the agenda buffer does not use font-lock for fontifying and instead inserts already fontified text in the buffer directly, we can't simply add a regexp with font-lock-add-keywords. But the solution is nonetheless very straight-forward. Create a function which will search for the regexp in the buffer and add the face text property. Then add it to the org-agenda-finalize-hook and that's that!

(defun my-fontify-progress-cookie ()
  "Fontify progress cookies in org agenda."
  (save-excursion
    (goto-char (point-min))
    (while (re-search-forward "\\[[[:digit:]]+/[[:digit:]]+\\]" nil t)
      (add-face-text-property (match-beginning 0) (match-end 0)
                              'org-checkbox-statistics-todo))
    (goto-char (point-min))
    (while (re-search-forward "\\[[[:digit:]]+%\\]" nil t)
      (add-face-text-property (match-beginning 0) (match-end 0)
                              'org-checkbox-statistics-todo))))

(add-hook 'org-agenda-finalize-hook 'my-fontify-progress-cookie)