Add metadata in content snippet

[padvincenzo]

Hi, I've ported the metadata in content snippet, following @alex40724 's patch.

Also, many unwanted spaces have been removed.

[kergomard]

Dear @alex40724 While checking on old open PRs in the frame of our Technical Board Meeting we stumbled over this one that is getting a little long in the tooth. Could you please either close it with an explanation, give feedback or ask @padvincenzo to resolve the conflicts and merge? Thank you very much! @kergomard in the name of the TB

[alex40724]

Again, sorry for the delay. This feature has only general acceptance for release 8+, see https://docu.ilias.de/goto_docu_wiki_wpage_6553_1357.html We can't backport this to the official 6 release branch.