[FEATURE] expose metric for current image creation timestamp & current image available

**Is your feature request related to a problem? Please describe.**
Having the image creation timestamp as a metric would be useful and help to discover images in use that are old/abandoned, that may already be at the latest. For example, an image already at latest that was created X months/years ago - can be flagged as a potential abandoned/image that needs investigation or a new version to be built.

The second part of this is it would double as a way to know if the current image no longer exists on the repo which would be useful to avoid potential image pull errors in the future. I could see that either being implemented as separate metric (version_checker_current_is_available), or by setting the proposed image creation timestamp metric to an invalid value, like null/0 and use that as a way to detect whether the image is not available on the remote repo. #319 is somewhat related (registry accessible)

**Describe the solution you'd like**
Expose the current versions creation timestamp (queried from remote reg)

**Describe alternatives you've considered**
Trying to find another tool to handle this.

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**Message from the maintainers**:

If you wish to see this enhancement implemented please add a 👍 reaction to this issue! We often sort issues this way to know what to prioritize.

No file or test is named. Start by tracing the existing image-version metrics and the remote registry lookup used by version-checker. Define how the current image creation timestamp and unavailable-image state should be represented, then verify that both cases are exposed consistently as metrics.

索引モデルが issue の本文から書いたものです。