Adding "URL" accessed to Apache Error log

taken from the debian apache2.conf default file:

# The following directives define some format nicknames for use with
# a CustomLog directive (see below).
# If you are behind a reverse proxy, you might want to change %h into %{X-Forwarded-For}i
#
LogFormat "%v:%p %h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" vhost_combined
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
LogFormat "%h %l %u %t \"%r\" %>s %b" common
LogFormat "%{Referer}i -> %U" referer
LogFormat "%{User-agent}i" agent

See Apache 2 Manual for the meaning of the different format signs % or Apache 2 Manual ErrorlogFormat Directive for having the error log in a specific format since apache 2.4 aswell.

in short :

Define the output format in your main config file and the output file for example per vhost in the VirtualHost Directive. You can define the config file for all your sites in the main file aswell, if you want to.

something like would be needed to add (in case its not there yet):

    LogLevel warn

    CustomLog /var/log/apache2/access.log combined
    ErrorLog /var/log/apache2/error.log

Depending on your exact setup you might want to change the location slightly.

Dont forgot to logrotate them if you have a high traffic site.

Normally this should be (at least in debian) the default setting for how i remember it.

If you have empty logs you might want to check the permissions of the folder/files so that the user the apache2 process runs under can access and write to them.

Remember a reload of the process is (for what i know) needed for rereading the configuration files.

The accepted answer does not answer the original question as it logs to access.log and not to error.log

you need to put the url in an environment variable and then call it from ErrorLogFormat like this:

SetEnvIf Request_URI "(^.*$)" RURI=$1
ErrorLogFormat "%{cu}t %a %l %M URI:%{RURI}e"

This worked for me:

ErrorLogFormat "[%t] %{Request_URI}e (Referer: %{Referer}i) [client %a] [%l] %M"

When I created a separate variable for the request URI as shown by @kofifus and @franfran, I got the underlying script that was running after URL rewriting, but I wanted the path as seen by the user in a web browser and that's what this gives me.