How to log the selected director>backend in varnishncsa

Question

I have multiple backend servers and I "round-robin" between them using a director.

Is there a way and - if there is - how to log which backend gets used (either the backend name or the backend host name)?
The above refers to using std.log("key:value") and %{VCL_Log:*key*}x with varnishncsa.

My vcl config:

backend aws_frontend1 {
  .host = "aws1.domain.mobi";
  .port = "80";
}
backend aws_frontend2 {
  .host = "aws2.domain.mobi";
  .port = "80";
}

director lb_aws_frontend round-robin {
  {
    .backend = aws_frontend1;
  }
  {
    .backend = aws_frontend2;
  }
}

sub vcl_recv {
  set req.backend = lb_aws_frontend;
  unset req.http.Cookie;
}
sub vcl_fetch {
  if (beresp.http.cache-control ~ "(no-cache|private)" || beresp.http.pragma ~ "no-cache") {
    set beresp.ttl = 0s;
  } else {
    set beresp.ttl = 168h;
  }
}
sub vcl_error {
  if (obj.status == 750) {
    set obj.status = 403;
    return(deliver);
  }
}

Edit:

Below is the updated code thanks to NITEMAN... This code prints out the backend name when a miss and "varnish cache" is a hit:

import std;

backend aws_frontend1 {
  .host = "aws1.domain.mobi";
  .port = "80";
}
backend aws_frontend2 {
  .host = "aws2.domain.mobi";
  .port = "80";
}

director lb_aws_frontend round-robin {
  {
    .backend = aws_frontend1;
  }
  {
    .backend = aws_frontend2;
  }
}

sub vcl_recv {
  set req.backend = lb_aws_frontend;
  unset req.http.Cookie;
}
sub vcl_fetch {
  if (beresp.http.cache-control ~ "(no-cache|private)" || beresp.http.pragma ~ "no-cache") {
    set beresp.ttl = 0s;
  } else {
    set beresp.ttl = 168h;
  }
  std.log("backend_used:" + beresp.backend.name);
}
sub vcl_hit {
  std.log("backend_used:varnish cache");
}
sub vcl_error {
  if (obj.status == 750) {
    set obj.status = 403;
    return(deliver);
  }
}

Answer 1

Backend's name (even when a director is used) is available on vcl_fetch, for debugging purposes I usually use:

sub vcl_fetch {
  # ...
  set beresp.http.X-Backend = beresp.backend.name;
  # ...
}