Allow setting a custom EventListener in ApiClientBuilder (#1621)

Hi Team,

Thank you for your great work on developing the SDK for LINE Bot in
Java.

I noticed that you have already implemented `HttpInterceptor` interface,
which allows the SDK users to add a custom `Interceptor` to the
underlying OKHttpClient.
(https://square.github.io/okhttp/features/interceptors/) With this
support, we can easily add our own custom `HttpInterceptor` or reuse
existing `OKHttpClient's Interceptor` by wrapping it accordingly.

I was wondering if it would be possible to also allow setting a custom
`EventListener` on the underlying
`OkHttpClient`.(https://square.github.io/okhttp/features/events/)

Based on our discussion, rather than introducing a new interface like
HttpInterceptor, it seems preferable to expose the EventListener
directly, as it is easier for users to use and also leads to a simpler
implementation.

This pull request introduces a proof of concept for adding such support.
The implementation closely mirrors that of the custom interceptor
mechanism , although `EventListener` is a bit more involved than
`Interceptor`.

With this feature, SDK users will be able to set a custom EventListener
to suit their needs when creating the API client. For Example:
```java
MessagingApiClient.builder("your access token")
                  .apiEndPoint(URI.create(lineApiUrl))
                  .addInterceptor(new MyHttpInterceptor)
                  .setEventListener(new MyEventListener())
                  .build();
```

My use case for `OKHttpClient's EventListener`.
In our system, we also use `OkHttpClient` along with `Retrofit`. To
enhance the observability of API calls in our system, we leverage the
`OkHttpMetricsEventListener` provided by `Micrometer` to record HTTP
request metrics.
(https://docs.micrometer.io/micrometer/reference/reference/okhttpclient.html).

We would like to attach this metrics to LINE Bot SDK's client by
adapting a `Micrometer`'s `OkHttpMetricsEventListener`, like this:
```java
MessagingApiClient.builder("your access token")
                  .apiEndPoint(URI.create(lineApiUrl))
                  .addInterceptor(new MyHttpInterceptor)
                  .setEventListener(new OkHttpMetricsEventListener())
                  .build();
```
Thank you for considering this suggestion. I look forward to your
feedback!

Author: kabigon-sung

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
                 + '}';