Set cookie in 10% of traffic

This example should help you: https://devcentral.f5.com/s/articles/ratio-load-balancing-using-rand-function

 

Just use if { rand() < 0.1 } { ...

 

 

It will be good if you throw some more light here. I tried to prepare irule based on above conditions. you can edit & apply based on condition.

when HTTP_REQUEST {
if { rand() < 0.1 } {
    if { [HTTP::cookie exists "old-cookie-name"] } {
   Insert a new cookie with the new name and old cookie's value
    HTTP::cookie insert name "new-cookie-name" value [HTTP::cookie value "old-cookie-name"]
   Remove the old cookie
  HTTP::cookie remove "old-cookie-name" }
    }
}