feat: add response time histogram metric

doc/fr/references/metrics.md

@@ -24,6 +24,31 @@ Chaque layer associé à un proxy peut également ses propres métriques spécif
   bouncer_proxy_director_proxy_requests_total{proxy="cadoles"} 64
   ```
 
+#### `bouncer_proxy_director_proxy_responses_duration_seconds{proxy=<proxyName>}`
+
+- **Type:** `histogram`
+- **Description**: Histogramme du temps de traitement du cycle requête/réponse par proxy
+- **Exemple**
+
+  ```
+  # HELP bouncer_proxy_director_proxy_responses_duration_seconds Bouncer proxy responses duration
+  # TYPE bouncer_proxy_director_proxy_responses_duration_seconds histogram
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.005"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.01"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.025"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.05"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.1"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.25"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="0.5"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="1"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="2.5"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="5"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="10"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_bucket{proxy="dummy",le="+Inf"} 13
+  bouncer_proxy_director_proxy_responses_duration_seconds_sum{proxy="dummy"} 0.009556884
+  bouncer_proxy_director_proxy_responses_duration_seconds_count{proxy="dummy"} 13
+  ```
+
 ### Serveur d'administration
 
 _Pas de métrique supplémentaire._