Deprecate node['gitlab'] monitoring attributes

Fixes: #4522 (closed)

Deprecate node['gitlab'] monitoring attributes instead of removal

This introduces a deprecation proxy object that is based on what is provided in chef for instance variables: https://github.com/chef/chef/blob/master/lib/chef/mixin/deprecation.rb#L61

It will log the deprecation message when you call into node['gitlab']['prometheus'], but will also return you what you might be looking for from monitoring in the meantime.

Before choosing this approach I did spent a lot of time trying to work node deprecations into our current list of deprecations. Where they were listed with our others, and things like which version they were deprecated in vs removed was honoured. I have the MR for that here: !3572 (closed) But ultimately it turned out too complicated of a solution vs doing this.