Expose information about registries as metrics

[adberger]

Comprehensive Summary of your change

With this change all registries are exposed as metrics in the harbor-exporter component.

This looks like this:

# HELP harbor_registry_info Information about the registry
# TYPE harbor_registry_info gauge
harbor_registry_info{description="",insecure="false",name="dockerhub",status="healthy",type="docker-hub",url="https://hub.docker.com"} 1

Issue being fixed

Fixes #17531 partially

Website update

goharbor/website#591

Dislaimer

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.