Prometheus metric for repository's last update

[oxzi]

What is the purpose of this change? What does it change?

A new gauge metric rest_server_repo_last_update_timestamp was added to monitor each repository's last write access. This allows a basic monitoring for each repository's freshness.

Was the change discussed in an issue or in the forum before?

This might be related to the feature request in #176.

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [x] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review

[oxzi]

@wojas: Thanks a lot for your review! I have made some changes and answered in the conversations above.

Furthermore, I tried to motivate why I introduced the preloading, which is not directly part of those metrics, but are imho necessary for a complete view after (automatically) restarting the rest-server.

Please feel free to take another look; and thanks again!

[riotbib]

This is relevant to me, please consider merging. Thanks!