Cookie Counting

If by "session", you mean TCP connection, there is an alternate way to accomplish this:

    when CLIENT_ACCEPTED {
        set client "[IP::client_addr]:[TCP::client_port]"
    }
    
    when HTTP_REQUEST {
        if { ![info exists ccount] } {
            set ccount [HTTP::cookie count]
        } else {
            incr ccount [HTTP::cookie count]
        }
        
        if { [HTTP::header exists User-Agent] } {
            set ua [HTTP::header User-Agent]
        }
    }

    when SERVER_CLOSED {
        if { ![info exists ua] } {
            set ua ""
        }
        
        log local0. "Client: ($client), Cookies: ($ccount), UA: ($ua)"
    }

This assumes that the User-Agent doesn't change over the lifetime of the TCP connection, which is generally a valid assumption.