[FEATURE] Expose release dates of current and latest releases in metrics

**Is your feature request related to a problem? Please describe.**

In my-use case I find that version-checker reports most images as behind out of date (expected for now..), which means I need to use some other data to help determine what order we go after upgrades.

**Describe the solution you'd like**

It would be great if the metrics could expose release dates for the current and latest version of the app. I could then diff this and create an ordering (based on how out of date the release is).

Semver alone can't really offer the same thing here, because it depends on the release strategy of the application in question, plus you can't just diff the 2 semver versions to get a count of releases in between.

I think i would favour date difference over number-of-releases-behind (when looking at current vs latest).

**Describe alternatives you've considered**

https://outdated.sh/ is interesting, but I think it's only a cli tool.

Thanks!

No files, tests, or entry points are named in the issue. Start by locating the existing metrics for current and latest versions and how release metadata is obtained; done means metrics expose release dates for both versions so their age difference can be used for upgrade ordering.

索引モデルが issue の本文から書いたものです。