Counting anonymous users with Varnish

Hi guys

I have a feature request, but that may be a support request. The function admin_menu_session_count() counts number of anonymous and authenticated users based on the entries in the sessions table. But when using a reverse proxy like Varnish, most of the anonymous page views are served by the RP, without any boostraping of Drupal. So the number of anonymous users displayed in the right corner of the admin menu bar is false.

I thought about several solutions :

1. Hacking the module (booh!) to perform a simple calculation in order to adjust the number of an. sessions. Code would look like this :
   

   - $count_anon = admin_menu_session_count($interval, TRUE);
   + $count_anon = $rp * admin_menu_session_count($interval, TRUE);
   // $rp is an estimation of the ratio of cache hits/cache misses based on Varnish stats
   

   I'm clearly not in favor of this custom hack, bad practice.

2. Have a custom configuration outside of Drupal, with an ESI for instance, where a small javascript bootstraps Drupal and add an entry in the session table. For high traffic website, it would also mean using something like RabbitMQ or another queue system to pile up the anonymous cache hits and write it back to the session table once a minute for instance.
   A performance problem because one RP cache hit = one entry in the session table
3. Have a similar configuration than point number 2, but write the count in another table (just a timestamp and number of hits in each row for instance, it's lighter since we only need the count and a timestamp for an approximate timerange) and allow admin_menu to query this table instead of the normal session table
   It's been the second company where some custom solution with this other table has been put in place. So I really think this would be the best solution for admin_menu to take this kind of solution into account.

So I'm in favor of solution 3. Clearly I understand that it's not really the problem of admin_menu and more the problem of how Drupal core handles visit statistics, but if it could just allow for a custom query instead of the query against the sessions table from the admin_menu_session_count() function it would already "officially" enable this kind of solution. The implementation I've thought would be another setting (stored in the variables table) that would allow through the UI to write this custom query. I know it's dirty and insecure. Another way would be to allow this query to be written in the code, through the use of a hook or something equivalent.

Long issue I'm sorry. But the goal would be to show more accurate anonymous stats through reverse proxies. What do you think ?