Add noop listener.

Author: chickenchickenlove

core/src/main/java/com/linecorp/armeria/client/ClientFactoryBuilder.java

@@ -136,8 +136,8 @@ public final class ClientFactoryBuilder implements TlsSetters {
     private ClientTlsConfig tlsConfig;
     private boolean staticTlsSettingsSet;
     private boolean autoCloseConnectionPoolListener = true;
-    private ClientRequestLifecycleListener clientRequestLifecycleListener = 
-            new ClientRequestMetrics();
+    private ClientRequestLifecycleListener clientRequestLifecycleListener =
+            ClientRequestLifecycleListener.noop();
 
     ClientFactoryBuilder() {
         connectTimeoutMillis(Flags.defaultConnectTimeoutMillis());

core/src/main/java/com/linecorp/armeria/client/metric/ClientRequestLifecycleListener.java

@@ -42,4 +42,43 @@ static ClientRequestMetrics counting() {
         return new ClientRequestMetrics();
     }
 
+    static ClientRequestLifecycleListener noop() {
+        return NoopClientRequestLifecycleListener.INSTANCE;
+    }
+    
+    class NoopClientRequestLifecycleListener implements ClientRequestLifecycleListener {
+        
+        private static final NoopClientRequestLifecycleListener INSTANCE = new NoopClientRequestLifecycleListener();
+        
+        @Override
+        public void onRequestPending(ClientRequestContext ctx) {
+            // no-op
+        }
+
+        @Override
+        public void onRequestStart(ClientRequestContext ctx) {
+            // no-op
+        }
+
+        @Override
+        public void onRequestSendComplete(ClientRequestContext ctx) {
+            // no-op
+        }
+
+        @Override
+        public void onResponseHeaders(ClientRequestContext ctx, ResponseHeaders headers) {
+            // no-op
+        }
+
+        @Override
+        public void onResponseComplete(ClientRequestContext ctx) {
+            // no-op
+        }
+
+        @Override
+        public void onRequestComplete(ClientRequestContext ctx, @Nullable Throwable cause) {
+            // no-op
+        }
+    }
+    
 }