GH-6372: Expose client request metrics.

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

@@ -53,6 +53,7 @@
 import com.google.common.collect.ImmutableMap;
 import com.google.common.primitives.Ints;
 
+import com.linecorp.armeria.client.metric.ClientRequestLifecycleListener;
 import com.linecorp.armeria.client.proxy.ProxyConfig;
 import com.linecorp.armeria.client.proxy.ProxyConfigSelector;
 import com.linecorp.armeria.common.CommonPools;
@@ -135,6 +136,8 @@ public final class ClientFactoryBuilder implements TlsSetters {
     private ClientTlsSpec clientTlsSpec = ClientTlsSpec.of();
     private boolean staticTlsSettingsSet;
     private boolean autoCloseConnectionPoolListener = true;
+    private ClientRequestLifecycleListener clientRequestLifecycleListener =
+            ClientRequestLifecycleListener.noop();
 
     ClientFactoryBuilder() {
         connectTimeoutMillis(Flags.defaultConnectTimeoutMillis());
@@ -261,6 +264,21 @@ public <T> ClientFactoryBuilder channelOption(ChannelOption<T> option, T value)
         return this;
     }
 
+    /**
+     * Sets the {@link ClientRequestLifecycleListener} that listens to the lifecycle events of
+     * client requests created by the {@link ClientFactory}.
+     *
+     * <p>If not set, {@link ClientRequestLifecycleListener#noop()} is used by default.
+     *
+     * @param listener the listener to be notified of client request lifecycle events. Must not be {@code null}.
+     * @return {@code this} to support method chaining.
+     */
+    public ClientFactoryBuilder clientRequestLifecycleListener(ClientRequestLifecycleListener listener) {
+        requireNonNull(listener, "listener");
+        clientRequestLifecycleListener = listener;
+        return this;
+    }
+
     private void channelOptions(Map<ChannelOption<?>, Object> newChannelOptions) {
         @SuppressWarnings("unchecked")
         final ClientFactoryOptionValue<Map<ChannelOption<?>, Object>> castOptions =
@@ -1127,7 +1145,7 @@ public ClientFactory build() {
         final ClientFactoryOptions options = buildOptions();
         final ClientTlsSpec baseClientTlsSpec = buildTlsSpec(clientTlsSpec, tlsNoVerifySet, insecureHosts);
         return new DefaultClientFactory(new HttpClientFactory(
-                options, autoCloseConnectionPoolListener, baseClientTlsSpec));
+                options, autoCloseConnectionPoolListener, baseClientTlsSpec, clientRequestLifecycleListener));
     }
 
     private static ClientTlsSpec buildTlsSpec(

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

@@ -104,6 +104,12 @@ public HttpResponse execute(ClientRequestContext ctx, HttpRequest req) throws Ex
         final DecodedHttpResponse res = new DecodedHttpResponse(eventLoop);
         updateCancellationTask(ctx, req, res);
 
+        factory.clientRequestLifecycleListener().onRequestPending(ctx);
+        ctx.log().addListener(
+                factory.clientRequestLifecycleListener()
+                        .asRequestLogListener()
+        );
+
         try {
             resolveProxyConfig(protocol, endpoint, ctx, (proxyConfig, thrown) -> {
                 if (thrown != null) {

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

@@ -38,6 +38,7 @@
 import com.google.common.collect.MapMaker;
 
 import com.linecorp.armeria.client.endpoint.EndpointGroup;
+import com.linecorp.armeria.client.metric.ClientRequestLifecycleListener;
 import com.linecorp.armeria.client.proxy.ProxyConfigSelector;
 import com.linecorp.armeria.client.redirect.RedirectConfig;
 import com.linecorp.armeria.common.Http1HeaderNaming;
@@ -123,15 +124,23 @@ final class HttpClientFactory implements ClientFactory {
     private final boolean autoCloseConnectionPoolListener;
     private final AsyncCloseableSupport closeable = AsyncCloseableSupport.of(this::closeAsync);
     private final BootstrapSslContexts bootstrapSslContexts;
+    private final ClientRequestLifecycleListener clientRequestLifecycleListener;
 
     HttpClientFactory(ClientFactoryOptions options, boolean autoCloseConnectionPoolListener,
                       ClientTlsSpec baseClientTlsSpec) {
+        this(options, autoCloseConnectionPoolListener, baseClientTlsSpec,
+                ClientRequestLifecycleListener.noop());
+    }
+
+    HttpClientFactory(ClientFactoryOptions options, boolean autoCloseConnectionPoolListener,
+                      ClientTlsSpec baseClientTlsSpec,
+                      ClientRequestLifecycleListener clientRequestLifecycleListener) {
         workerGroup = options.workerGroup();
 
         @SuppressWarnings("unchecked")
         final AddressResolverGroup<InetSocketAddress> group =
                 (AddressResolverGroup<InetSocketAddress>) options.addressResolverGroupFactory()
-                                                                 .apply(workerGroup);
+                        .apply(workerGroup);
         addressResolverGroup = group;
 
         final Bootstrap bootstrap = new Bootstrap();
@@ -199,6 +208,7 @@ final class HttpClientFactory implements ClientFactory {
         this.autoCloseConnectionPoolListener = autoCloseConnectionPoolListener;
 
         this.options = options;
+        this.clientRequestLifecycleListener = clientRequestLifecycleListener;
 
         clientDelegate = new HttpClientDelegate(this, addressResolverGroup);
         RequestTargetCache.registerClientMetrics(meterRegistry);
@@ -306,6 +316,10 @@ Http1HeaderNaming http1HeaderNaming() {
         return http1HeaderNaming;
     }
 
+    ClientRequestLifecycleListener clientRequestLifecycleListener() {
+        return clientRequestLifecycleListener;
+    }
+
     @VisibleForTesting
     AddressResolverGroup<InetSocketAddress> addressResolverGroup() {
         return addressResolverGroup;

core/src/main/java/com/linecorp/armeria/client/logging/DefaultClientRequestLogListener.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2025 LY Corporation
+ *
+ * LY Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.client.logging;
+
+import com.linecorp.armeria.client.ClientRequestContext;
+import com.linecorp.armeria.client.metric.ClientRequestLifecycleListener;
+import com.linecorp.armeria.common.annotation.UnstableApi;
+import com.linecorp.armeria.common.logging.RequestLog;
+import com.linecorp.armeria.common.logging.RequestLogListener;
+import com.linecorp.armeria.common.logging.RequestLogProperty;
+
+/**
+ * A default implementation of {@link RequestLogListener} that delegates {@link RequestLog} events
+ * to a {@link ClientRequestLifecycleListener}.
+ *
+ * <p>This listener monitors the progress of a {@link RequestLog} and invokes the corresponding
+ * callback methods of the {@link ClientRequestLifecycleListener} when specific {@link RequestLogProperty}
+ * become available.
+ */
+@UnstableApi
+public class DefaultClientRequestLogListener implements RequestLogListener {
+
+    private final ClientRequestLifecycleListener lifecycleListener;
+
+    /**
+     * Creates a new instance with the specified {@link ClientRequestLifecycleListener}.
+     *
+     * @param lifecycleListener the listener to which the {@link RequestLog} events will be delegated
+     */
+    public DefaultClientRequestLogListener(ClientRequestLifecycleListener lifecycleListener) {
+        this.lifecycleListener = lifecycleListener;
+    }
+
+    @Override
+    public void onEvent(RequestLogProperty property, RequestLog log) {
+        if (!(log.context() instanceof ClientRequestContext)) {
+            return;
+        }
+
+        final ClientRequestContext ctx = (ClientRequestContext) log.context();
+        switch (property) {
+            case REQUEST_FIRST_BYTES_TRANSFERRED_TIME:
+                lifecycleListener.onRequestStart(ctx);
+                break;
+            case REQUEST_COMPLETE:
+                lifecycleListener.onRequestSendComplete(ctx);
+                break;
+            case RESPONSE_HEADERS:
+                lifecycleListener.onResponseHeaders(ctx, log.responseHeaders());
+                break;
+            case RESPONSE_COMPLETE:
+                lifecycleListener.onResponseComplete(ctx);
+                break;
+            case ALL_COMPLETE:
+                lifecycleListener.onRequestComplete(ctx, log.responseCause());
+                break;
+            default:
+                break;
+        }
+    }
+}

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

@@ -0,0 +1,111 @@
+/*
+ * Copyright 2025 LY Corporation
+ *
+ * LY Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.client.metric;
+
+import com.linecorp.armeria.client.ClientRequestContext;
+import com.linecorp.armeria.client.logging.DefaultClientRequestLogListener;
+import com.linecorp.armeria.common.ResponseHeaders;
+import com.linecorp.armeria.common.annotation.Nullable;
+import com.linecorp.armeria.common.annotation.UnstableApi;
+import com.linecorp.armeria.common.logging.RequestLogListener;
+
+/**
+ * Listens to the lifecycle events of client requests.
+ *
+ * <p>This interface provides a high-level view of a request's lifecycle (e.g., pending, started, completed).
+ * unlike {@link RequestLogListener} which provides low-level property changes.
+ *
+ * <p>Note: The methods in this interface are typically not invoked directly by the
+ * transport layer. Instead, they are triggered by inspecting the changes in
+ * {@link com.linecorp.armeria.common.logging.RequestLog}.
+ * Implementations should use {@link #asRequestLogListener()} to bridge
+ * {@link com.linecorp.armeria.common.logging.RequestLogProperty}
+ * changes to these lifecycle methods.
+ *
+ * <p>For example, a standard implementation might map:
+ * <ul>
+ * <li>{@link com.linecorp.armeria.common.logging.RequestLogProperty#REQUEST_FIRST_BYTES_TRANSFERRED_TIME}
+ * to {@link #onRequestStart(ClientRequestContext)}</li>
+ * <li>{@link com.linecorp.armeria.common.logging.RequestLogProperty#ALL_COMPLETE}
+ * to {@link #onRequestComplete(ClientRequestContext, Throwable)}</li>
+ * </ul>
+ *
+ * @see com.linecorp.armeria.client.ClientFactoryBuilder#clientRequestLifecycleListener(
+ * ClientRequestLifecycleListener)
+ */
+@UnstableApi
+public interface ClientRequestLifecycleListener {
+
+    /**
+     * Invoked when a request is created and scheduled for execution but has not yet started.
+     * Note: This method is explicitly invoked by HttpClientDelegate when
+     * HttpClientDelegate starts to call execute().
+     */
+    void onRequestPending(ClientRequestContext ctx);
+
+    /**
+     * Called when the client begins execution (connection acquired, headers sent).
+     */
+    void onRequestStart(ClientRequestContext ctx);
+
+    /**
+     * Called when the request is fully sent.
+     */
+    void onRequestSendComplete(ClientRequestContext ctx);
+
+    /**
+     * Called when the first response headers are received.
+     */
+    void onResponseHeaders(ClientRequestContext ctx, ResponseHeaders headers);
+
+    /**
+     * Called when the full response body is received successfully.
+     */
+    void onResponseComplete(ClientRequestContext ctx);
+
+    /**
+     * Called when a request is completed with either success or failure.
+     */
+    void onRequestComplete(ClientRequestContext ctx, @Nullable Throwable cause);
+
+    /**
+     * Returns a {@link RequestLogListener} that delegates
+     * {@link com.linecorp.armeria.common.logging.RequestLog}
+     * events to this lifecycle listener.
+     * This method bridges the low-level {@link com.linecorp.armeria.common.logging.RequestLog}
+     * updates to the high-level lifecycle methods
+     * defined in this interface. The returned listener is registered to the {@link ClientRequestContext}
+     * automatically when the request starts.
+     */
+    default RequestLogListener asRequestLogListener() {
+        return new DefaultClientRequestLogListener(this);
+    }
+
+    /**
+     * Returns a {@link ClientRequestMetrics} that collects the number of pending and active requests.
+     */
+    static ClientRequestMetrics counting() {
+        return new ClientRequestMetrics();
+    }
+
+    /**
+     * Returns a {@link ClientRequestLifecycleListener} that does nothing.
+     */
+    static ClientRequestLifecycleListener noop() {
+        return NoopClientRequestLifecycleListener.getInstance();
+    }
+}