extract tags from nginx variables prefixed by `pinba_tag_`

Full example:

    location ~^/track/(.*)$ {
      pinba_enable on;
      pinba_request_timer t;
      pinba_max_timer 2;
      set $pinba_request_uri $1;
      set $pinba_tag_as 3215;
      set $pinba_tag_country FR;
      set $pinba_tag_browser $http_user_agent;
      return 204;
    }