add config logging on agent startup

custom/build.gradle.kts

@@ -75,7 +75,11 @@ tasks {
   }
 }
 
-tasks.withType<Test> {
+tasks.withType<Test>().configureEach {
+  val agentJarTask = project(":agent").tasks.named<Jar>("jar")
+  dependsOn(agentJarTask)
+  systemProperty("elastic.otel.agent.jar.path", agentJarTask.get().archiveFile.get().asFile.absolutePath)
+
   val overrideConfig = project.properties["elastic.otel.overwrite.config.docs"]
   if (overrideConfig != null) {
     systemProperty("elastic.otel.overwrite.config.docs", overrideConfig)

custom/src/main/java/co/elastic/otel/ElasticAutoConfigurationCustomizerProvider.java

@@ -19,6 +19,7 @@
 package co.elastic.otel;
 
 import co.elastic.otel.compositesampling.DynamicCompositeParentBasedTraceIdRatioBasedSampler;
+import co.elastic.otel.config.ConfigLoggingAgentListener;
 import co.elastic.otel.dynamicconfig.BlockableLogRecordExporter;
 import co.elastic.otel.dynamicconfig.BlockableMetricExporter;
 import co.elastic.otel.dynamicconfig.BlockableSpanExporter;
@@ -104,6 +105,8 @@ static Map<String, String> propertiesCustomizer(ConfigProperties configPropertie
     deltaMetricsTemporality(config, configProperties);
     resourceProviders(config, configProperties);
     spanStackTrace(config, configProperties);
+    ConfigLoggingAgentListener.logTheConfig(
+        configProperties.getBoolean(ConfigLoggingAgentListener.LOG_THE_CONFIG, true));
 
     return config;
   }

custom/src/main/java/co/elastic/otel/compositesampling/DynamicCompositeParentBasedTraceIdRatioBasedSampler.java

@@ -32,7 +32,8 @@ public enum DynamicCompositeParentBasedTraceIdRatioBasedSampler implements Sampl
 
   static final double DEFAULT_TRACEIDRATIO_SAMPLE_RATIO = 1.0d;
 
-  private static Sampler delegate = newSampler(DEFAULT_TRACEIDRATIO_SAMPLE_RATIO);
+  private static volatile double latestRatio;
+  private static volatile Sampler delegate = newSampler(DEFAULT_TRACEIDRATIO_SAMPLE_RATIO);
 
   public static void setRatio(double ratio) {
     delegate = newSampler(ratio);
@@ -55,6 +56,15 @@ public String getDescription() {
   }
 
   private static Sampler newSampler(double ratio) {
+    latestRatio = ratio;
     return ConsistentSampler.parentBased(ConsistentSampler.probabilityBased(ratio));
   }
+
+  public String toString() {
+    return "DynamicCompositeParentBasedTraceIdRatioBasedSampler(ratio="
+        + latestRatio
+        + ", "
+        + delegate.toString()
+        + ")";
+  }
 }

custom/src/main/java/co/elastic/otel/config/ConfigLoggingAgentListener.java

@@ -0,0 +1,49 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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
+ *
+ *   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 co.elastic.otel.config;
+
+import com.google.auto.service.AutoService;
+import io.opentelemetry.javaagent.extension.AgentListener;
+import io.opentelemetry.sdk.autoconfigure.AutoConfiguredOpenTelemetrySdk;
+import java.util.logging.Logger;
+
+@AutoService(AgentListener.class)
+public class ConfigLoggingAgentListener implements AgentListener {
+  public static final String LOG_THE_CONFIG =
+      "elastic.otel.java.experimental.configuration.logging.enabled";
+  private static final Logger logger = Logger.getLogger(ConfigLoggingAgentListener.class.getName());
+
+  private static boolean logTheConfig = true;
+
+  public static synchronized void logTheConfig(boolean logTheConfig) {
+    ConfigLoggingAgentListener.logTheConfig = logTheConfig;
+  }
+
+  @Override
+  public void afterAgent(AutoConfiguredOpenTelemetrySdk autoConfiguredOpenTelemetrySdk) {
+    if (logTheConfig) {
+      logger.info(autoConfiguredOpenTelemetrySdk.toString());
+    }
+  }
+
+  @Override
+  public int order() {
+    return 1;
+  }
+}

custom/src/test/java/co/elastic/otel/config/ConfigLoggingAgentListenerTest.java

@@ -0,0 +1,163 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. 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
+ *
+ *   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 co.elastic.otel.config;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.BufferedWriter;
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+public class ConfigLoggingAgentListenerTest {
+  static final String TARGET_CLASS_NAME = "TempTest321";
+  static String[] identifyingStrings = {
+    "ConfigLoggingAgentListener - AutoConfiguredOpenTelemetrySdk{",
+    "tracerProvider=SdkTracerProvider"
+  };
+
+  private static String agentJarFile;
+  private static File testTargetClass;
+
+  @BeforeAll
+  public static void setup() throws IOException {
+    agentJarFile = getAgentJarFile();
+    testTargetClass = createTestTarget();
+  }
+
+  @AfterAll
+  public static void teardown() throws IOException {
+    testTargetClass.delete();
+  }
+
+  @Test
+  public void checkLogConfigPresent() throws IOException {
+    String output = executeCommand(createTestTargetCommand(true), 20);
+    for (String identifyingString : identifyingStrings) {
+      assertThat(output).contains(identifyingString);
+    }
+  }
+
+  @Test
+  public void checkLogConfigAbsent() throws IOException {
+    String output = executeCommand(createTestTargetCommand(false), 20);
+    for (String identifyingString : identifyingStrings) {
+      assertThat(output).doesNotContain(identifyingString);
+    }
+  }
+
+  static List<String> createTestTargetCommand(boolean logConfig) throws IOException {
+    List<String> command = new ArrayList<>();
+    command.add("java");
+    command.add("-Xmx32m");
+    command.add("-javaagent:" + agentJarFile);
+    // Only on false, ie test the 'true' default with no option
+    if (!logConfig) {
+      command.add("-D" + ConfigLoggingAgentListener.LOG_THE_CONFIG + "=false");
+    }
+    command.add(testTargetClass.getAbsolutePath());
+    return command;
+  }
+
+  static File createTestTarget() throws IOException {
+    String targetClass =
+        "public class "
+            + TARGET_CLASS_NAME
+            + " {\n"
+            + "  public static void main(String[] args) {\n"
+            + "    System.out.println(\"Test started\");\n"
+            // 15 seconds to give the agent plenty of time to start and be fully initialized
+            + "    try {Thread.sleep(15_000L);} catch (InterruptedException e) {}\n"
+            + "    System.out.println(\"Test ended\");\n"
+            + "  }\n"
+            + "}\n";
+    File targetClassFile =
+        new File(System.getProperty("java.io.tmpdir"), TARGET_CLASS_NAME + ".java");
+    targetClassFile.deleteOnExit();
+    try (BufferedWriter writer = new BufferedWriter(new FileWriter(targetClassFile))) {
+      writer.write(targetClass);
+    }
+    return targetClassFile;
+  }
+
+  static String getAgentJarFile() {
+    String path = System.getProperty("elastic.otel.agent.jar.path");
+    if (path == null) {
+      throw new IllegalStateException("elastic.otel.agent.jar.path system property is not set");
+    }
+    return path;
+  }
+
+  public static String executeCommand(List<String> command, int timeoutSeconds) throws IOException {
+    ProcessBuilder pb = new ProcessBuilder(command);
+    pb.redirectErrorStream(true);
+    Process childProcess = pb.start();
+
+    StringBuilder commandOutput = new StringBuilder();
+
+    boolean isAlive = true;
+    byte[] buffer = new byte[64 * 1000];
+    InputStream in = childProcess.getInputStream();
+    // stop trying if the time elapsed exceeds the timeout
+    while (isAlive && (timeoutSeconds > 0)) {
+      while (in.available() > 0) {
+        int lengthRead = in.read(buffer, 0, buffer.length);
+        commandOutput.append(new String(buffer, 0, lengthRead));
+      }
+      pauseSeconds(1);
+      timeoutSeconds--;
+      isAlive = childProcess.isAlive();
+    }
+    // it can die but still have output available buffered
+    while (in.available() > 0) {
+      int lengthRead = in.read(buffer, 0, buffer.length);
+      commandOutput.append(new String(buffer, 0, lengthRead));
+    }
+
+    // Cleanup as well as I can
+    boolean exited = false;
+    try {
+      exited = childProcess.waitFor(3, TimeUnit.SECONDS);
+    } catch (InterruptedException e) {
+    }
+    if (!exited) {
+      childProcess.destroy();
+      pauseSeconds(1);
+      if (childProcess.isAlive()) {
+        childProcess.destroyForcibly();
+      }
+    }
+
+    return commandOutput.toString();
+  }
+
+  private static void pauseSeconds(int seconds) {
+    try {
+      Thread.sleep(seconds * 1_000L);
+    } catch (InterruptedException e) {
+    }
+  }
+}