fix: Add missing logging for MCP Servers

This is (very) useful for debugging MCP related problems.

I've stumbled over this while I've built https://docs.enola.dev/concepts/mcp.

The code is originally from (my) https://github.com/enola-dev/enola/blob/ffc004666ea7f71357562ef12464d2b9fdbf9dbd/java/dev/enola/ai/mcp/McpServer.java#L29, but I'm hereby donating it! ;-)

PiperOrigin-RevId: 796463552

Author: vorburger

core/src/main/java/com/google/adk/tools/mcp/McpServerLogConsumer.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed 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
+ *
+ * http://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.google.adk.tools.mcp;
+
+import io.modelcontextprotocol.spec.McpSchema;
+import io.modelcontextprotocol.spec.McpSchema.LoggingMessageNotification;
+import java.util.function.Consumer;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.slf4j.event.Level;
+
+class McpServerLogConsumer implements Consumer<LoggingMessageNotification> {
+
+  @Override
+  public void accept(LoggingMessageNotification notif) {
+    Logger log = LoggerFactory.getLogger(notif.logger());
+    log.atLevel(convert(notif.level())).log("{}", notif.data());
+  }
+
+  private Level convert(McpSchema.LoggingLevel level) {
+    return switch (level) {
+      case DEBUG -> Level.DEBUG;
+      case INFO, NOTICE -> Level.INFO;
+      case WARNING -> Level.WARN;
+      case ERROR, CRITICAL, ALERT, EMERGENCY -> Level.ERROR;
+    };
+  }
+}

core/src/main/java/com/google/adk/tools/mcp/McpSessionManager.java

@@ -71,6 +71,7 @@ public static McpSyncClient initializeSession(
             .initializationTimeout(
                 Optional.ofNullable(initializationTimeout).orElse(Duration.ofSeconds(10)))
             .requestTimeout(Optional.ofNullable(requestTimeout).orElse(Duration.ofSeconds(10)))
+            .loggingConsumer(new McpServerLogConsumer())
             .capabilities(ClientCapabilities.builder().build())
             .build();
     InitializeResult initResult = client.initialize();