Add thread dump debug option

apm-agent-core/src/main/java/co/elastic/apm/agent/configuration/CoreConfiguration.java

@@ -488,6 +488,13 @@ public String toString(Collection<String> value) {
             "exist and use it to dump bytecode of instrumented classes.")
         .buildWithDefault("");
 
+    private final ConfigurationOption<TimeDuration> threadDumpInterval = TimeDurationValueConverter.durationOption("ms")
+        .key("thread_dump_interval")
+        .configurationCategory(CORE_CATEGORY)
+        .tags("internal")
+        .description("Triggers a thread dump at regular frequency, should be used to help configure trace_methods without knowledge of application code")
+        .buildWithDefault(TimeDuration.of("0ms"));
+
     private final ConfigurationOption<Boolean> typeMatchingWithNamePreFilter = ConfigurationOption.booleanOption()
         .key("enable_type_matching_name_pre_filtering")
         .configurationCategory(CORE_CATEGORY)
@@ -1017,6 +1024,10 @@ public String getBytecodeDumpPath() {
         return bytecodeDumpPath.get();
     }
 
+    public long getThreadDumpInterval() {
+        return threadDumpInterval.get().getMillis();
+    }
+
     public boolean isTypeMatchingWithNamePreFilter() {
         return typeMatchingWithNamePreFilter.get();
     }

apm-agent-core/src/main/java/co/elastic/apm/agent/util/ThreadDump.java

@@ -0,0 +1,85 @@
+/*
+ * 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.apm.agent.util;
+
+import co.elastic.apm.agent.configuration.CoreConfiguration;
+import co.elastic.apm.agent.sdk.logging.Logger;
+import co.elastic.apm.agent.sdk.logging.LoggerFactory;
+import co.elastic.apm.agent.tracer.AbstractLifecycleListener;
+import co.elastic.apm.agent.tracer.Tracer;
+
+import javax.annotation.Nullable;
+import java.lang.management.ManagementFactory;
+import java.lang.management.ThreadInfo;
+import java.lang.management.ThreadMXBean;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+public class ThreadDump extends AbstractLifecycleListener {
+
+    private Logger log = LoggerFactory.getLogger(ThreadDump.class);
+
+    @Nullable
+    private ScheduledThreadPoolExecutor executor;
+
+    @Override
+    public void start(Tracer tracer) throws Exception {
+
+        long threadDumpInterval = tracer.getConfig(CoreConfiguration.class).getThreadDumpInterval();
+        if (threadDumpInterval <= 0) {
+            return;
+        }
+
+        if (!log.isDebugEnabled()) {
+            log.error("thread dump option requires debug log level");
+            return;
+        }
+
+        if (threadDumpInterval < 100) {
+            log.error("thread dump frequency too high, adjusted to every 100ms");
+            threadDumpInterval = 100;
+        }
+
+        log.warn("thread dump will be generated every %s ms", threadDumpInterval);
+
+        executor = ExecutorUtils.createSingleThreadSchedulingDaemonPool("thread-dump");
+
+        executor.scheduleAtFixedRate(new Runnable() {
+            @Override
+            public void run() {
+                ThreadMXBean threadMXBean = ManagementFactory.getThreadMXBean();
+                StringBuilder sb = new StringBuilder();
+                for (ThreadInfo threadInfo : threadMXBean.dumpAllThreads(true, true)) {
+                    sb.append(threadInfo.toString());
+                }
+
+                log.debug("thread dump: \n\n {}", sb);
+
+            }
+        }, threadDumpInterval, threadDumpInterval, TimeUnit.MILLISECONDS);
+    }
+
+    @Override
+    public void stop() throws Exception {
+        if (executor == null) {
+            return;
+        }
+        ExecutorUtils.shutdownAndWaitTermination(executor);
+    }
+}

apm-agent-core/src/main/resources/META-INF/services/co.elastic.apm.agent.tracer.LifecycleListener

@@ -10,3 +10,4 @@ co.elastic.apm.agent.metrics.builtin.AgentOverheadMetrics
 co.elastic.apm.agent.impl.circuitbreaker.CircuitBreaker
 co.elastic.apm.agent.collections.WeakMapCleaner
 co.elastic.apm.agent.report.serialize.MetricRegistryReporter
+co.elastic.apm.agent.util.ThreadDump