Add "pending updates" data to Prometheus Exporter

I monitoring Home Assistant via Prometheus, but the one thing I don’t appear to be able to do is to see when updates are available and how many custom components etc. need updating.

This information is available in various areas of the UI, so it would be great to get it exported via the Prometheus endpoint as well.

I’m happy to have a go at getting this working, but I’m struggling to find where in the codebase the various updates functions live (searching the core repo for “updates” returns over 1.4k results!).

The update entities likely make it easier to implement this feature. I was using the binary sensors that relate to (most) update entities, but these are going to be removed. See GitHub issue #71437.