Add header to requests on a sample rate

gistfile1.c

// at head of vcl
C{
    static __thread struct drand48_data random_seed;
    static long int sample_rate = 1000;
}C

// in vcl recv
C{
    long int r;
    lrand48_r(&random_seed, &r);
    if (0 == r % sample_rate) {
        // add header
    }
}C