Allow setting a custom EventListener in ApiClientBuilder

clients/line-bot-client-base/src/main/java/com/linecorp/bot/client/base/ApiClientBuilder.java

@@ -37,6 +37,7 @@
 import com.linecorp.bot.jackson.ModelObjectMapper;
 
 import okhttp3.Dispatcher;
+import okhttp3.EventListener;
 import okhttp3.Interceptor;
 import okhttp3.OkHttpClient;
 import okhttp3.Request;
@@ -89,6 +90,13 @@ public ApiClientBuilder(URI apiEndPoint, Class<T> clientClass, ExceptionBuilder
      */
     private List<Interceptor> additionalInterceptors = new ArrayList<>();
 
+    /**
+     * Custom EventListener
+     *
+     * <p>You can add your own EventListener.
+     */
+    private EventListener eventListener;
+
     private Proxy proxy;
 
     private HttpAuthenticator proxyAuthenticator;
@@ -141,6 +149,11 @@ public ApiClientBuilder<T> addInterceptor(HttpInterceptor interceptor) {
         return this;
     }
 
+    public ApiClientBuilder<T> setEventListener(EventListener eventListener) {
+        this.eventListener = eventListener;
+        return this;
+    }
+
     /**
      * The maximum number of requests to execute concurrently.
      * Default: 64
@@ -230,6 +243,10 @@ public T build() {
             }
         });
 
+        if (this.eventListener != null) {
+            okHttpClientBuilder.eventListener(this.eventListener);
+        }
+
         if (this.proxy != null) {
             okHttpClientBuilder.proxy(this.proxy);
         }
@@ -262,6 +279,7 @@ public String toString() {
                 + ", readTimeout=" + readTimeout
                 + ", writeTimeout=" + writeTimeout
                 + ", additionalInterceptors=" + additionalInterceptors
+                + ", eventListener=" + eventListener
                 + ", maxRequests=" + maxRequests
                 + ", maxRequestsPerHost=" + maxRequestsPerHost
                 + '}';