Added logic to unwind and surface the potential root cause when an error occurs during the agent's premain initialization phase.

Author: AlexeyKuznetsov-DD

dd-java-agent/src/main/java/datadog/trace/bootstrap/BootstrapInitializationTelemetry.java

@@ -6,6 +6,7 @@
 import java.io.OutputStream;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Stack;
 
 /** Thread safe telemetry class used to relay information about tracer activation. */
 public abstract class BootstrapInitializationTelemetry {
@@ -115,7 +116,23 @@ public void onAbort(String reasonCode) {
 
     @Override
     public void onError(Throwable t) {
-      onPoint("library_entrypoint.error", "error_type:" + t.getClass().getName());
+      Stack<String> causes = new Stack<>();
+
+      Throwable cause = t.getCause();
+      if (cause != null) {
+        while (cause != null) {
+          causes.push(cause.getClass().getName());
+          cause = cause.getCause();
+        }
+      }
+      causes.push(t.getClass().getName());
+
+      // Limit the number of causes to avoid overpopulating the JSON payload.
+      int cnt = Math.min(5, causes.size());
+      while (cnt > 0) {
+        onPoint("library_entrypoint.error", "error_type:" + causes.pop());
+        cnt--;
+      }
     }
 
     @Override

dd-java-agent/src/test/groovy/datadog/trace/bootstrap/BootstrapInitializationTelemetryTest.groovy

@@ -76,6 +76,18 @@ class BootstrapInitializationTelemetryTest extends Specification {
     !capture.json().contains("library_entrypoint.complete")
   }
 
+  def "unwind root cause"() {
+    when:
+    initTelemetry.initMetaInfo("runtime_name", "java")
+    initTelemetry.initMetaInfo("runtime_version", "1.8.0_382")
+
+    initTelemetry.onError(new Exception("top cause", new NullPointerException("root cause")))
+    initTelemetry.finish()
+
+    then:
+    capture.json() == '{"metadata":{"runtime_name":"java","runtime_version":"1.8.0_382"},"points":[{"name":"library_entrypoint.error","tags":["error_type:java.lang.Exception"]},{"name":"library_entrypoint.error","tags":["error_type:java.lang.NullPointerException"]},{"name":"library_entrypoint.complete"}]}'
+  }
+
   static class Capture implements BootstrapInitializationTelemetry.JsonSender {
     String json