BUG/MINOR: contrib/prometheus-exporter: Rename some metrics

BUG/MINOR: contrib/prometheus-exporter: Rename some metrics

The following metrics have been renamed without the "_http" part :

 * http_queue_time_average_seconds     => queue_time_average_seconds
 * http_connect_time_average_seconds   => connect_time_average_seconds
 * http_response_time_average_seconds  => response_time_average_seconds
 * http_total_time_average_seconds     => total_time_average_seconds

These metrics are reported per backend and per server and are not specific to
HTTP sessions.