Links in annotations - support relative URLs

What did you do?
Create two annotations with a url:
https://web.url.com/
/relative/url/path

What did you expect to see?
For both to be converted to a linkified URL link and displayed as the link icon

What did you see instead? Under which circumstances?
The first is converted because as #884 and the pull request #946 implemented it looks for things starting with http(s)://
The second obviously did not convert to a linkified URL and displays as a simple annotation. There should be a way to specify relative path URLS. This breaks implementations that are behind proxies that rewrite the FQDN of the URL and all other reasons for using relative versus absolute urls...

Please provide a way to permit relative URLs in annotations that will be linkified. Either a keyword in the annotation name or some type of markup to indicate that a url will follow in the annotation itself.

Environment
Prometheus 2.x

Review issue #884 and pull request #946 first to understand the existing annotation URL linkification. Then trace how Alertmanager handles annotations and define a supported way to mark relative URLs for linkification; done means both absolute and relative examples display as link icons without breaking existing behavior.