I’ve added a custom field, and want to use it on all posts.

I want to add a character counter to this custom field using functions.php. As far as I can tell, there’s no identifier to hook into. It seems the same field has a unique id from post to post, but no common value that I can use to target all of them. Here are two examples of the same custom field from two separate posts:

id="meta-6806-value"
id="meta-6809-value"

I can’t figure out how to target ALL of the same custom fields with a script. Am I missing something?