IIS 10 - CloudFlare Proxy Reversing (mod_remoteip or mod_cloudflare)
Solution 1:
You should be able to achieve this out of the box with IIS 10. Please refer to the following article https://support.cloudflare.com/hc/en-us/articles/200170786-How-do-I-correct-visitor-IP-with-Microsoft-IIS- (Webserver Instructions > Microsoft IIS section)
See also https://docs.microsoft.com/en-us/iis/get-started/whats-new-in-iis-85/enhanced-logging-for-iis85