try this one cool trick to do distributed tracing and RUM Injection via VCL

rum_injection.vcl

sub vcl_recv {
  declare local var.timemsec TIME;
  declare local var.traceid INTEGER;

  if(req.http.x-datadog-trace-id) {

  } else{
    set var.traceid = randomint(0, 2147483647);
    set var.traceid <<= 32;
    set var.traceid |= randomint(0, 4294967295);
    set req.http.X-Datadog-Trace-Id = var.traceid;
    set req.http.X-Datadog-Origin = "synthetics";
  }
  set req.http.X-Cdn-Start = time.start.msec;
}

sub vcl_deliver {
 if(req.http.X-Datadog-Trace-Id) {
    set resp.http.Server-Timing = req.http.X-Datadog-Trace-Id;
 }
}