Can't add tag to apache config or log files
The files are owned by root and Finder runs as the user, so Finder is not allowed to modify the tags.

As a workaround, you can run Finder as root:

sudo /System/Library/CoreServices/Finder.app/Contents/MacOS/Finder


Or use tag or xattr:

brew install tag;sudo tag -s $'Red\n6' /etc/apache2/httpd.conf

sudo xattr -w com.apple.metadata:_kMDItemUserTags '("Red\n6")' /etc/apache2/httpd.conf;sudo xattr -wx com.apple.FinderInfo '00 00 00 00 00 00 00 00 00 0C 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00' /etc/apache2/httpd.conf