set $gtm 'www.googletagmanager.com';
set $ga  'analytics.google.com';

location /gtm.js {
    sub_filter $gtm $server_name;
    sub_filter_types *;
    sub_filter_once off;

    proxy_set_header Accept-Encoding '';
    proxy_pass https://$gtm/gtm.js$is_args$args;
    proxy_redirect off;

    # cache on server
    proxy_cache ZONE0;
    proxy_cache_key $host$uri$is_args$args;
    proxy_cache_valid 200 304 60m;

    # cache in client browser
    expires 1h;
}

location /gtag/js {
    sub_filter $gtm $server_name;
    sub_filter $ga  $server_name;
    sub_filter_types *;
    sub_filter_once off;

    proxy_set_header Accept-Encoding '';

    proxy_pass https://$gtm/gtag/js$is_args$args;
    proxy_redirect off;
    proxy_cache ZONE0;
    proxy_cache_key $host$uri$is_args$args;
    proxy_cache_valid 200 304 60m;

    expires 1h;
}

location /g/collect {
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

    proxy_pass https://$ga$uri$is_args$args&uip=$remote_addr;
    proxy_redirect off;
    # proxy_cache ZONE0;
    # proxy_cache_key $host$uri$is_args$args;
    # proxy_cache_valid 200 304 60m;
    proxy_cache off;

    expires epoch; # browser no-cache control
}