chore(java-lang): Disable instrumentation by default

Author: PerfectSlayer

dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/main/java/datadog/trace/instrumentation/java/lang/jdk21/VirtualThreadInstrumentation.java

@@ -59,6 +59,12 @@ public boolean isEnabled() {
     return JavaVirtualMachine.isJavaVersionAtLeast(21) && super.isEnabled();
   }
 
+  @Override
+  protected boolean defaultEnabled() {
+    // Disabled by default until non flaky
+    return false;
+  }
+
   @Override
   public Map<String, String> contextStore() {
     Map<String, String> contextStore = new HashMap<>();

dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/test/groovy/VirtualThreadApiTest.groovy

@@ -6,6 +6,13 @@ import datadog.trace.test.util.Flaky
 // Note: test builder x2 + test factory can be refactored but are kept simple to ease with debugging.
 @Flaky("class loader deadlock on virtual thread clean up while Groovy do dynamic code generation - APMLP-782")
 class VirtualThreadApiTest extends InstrumentationSpecification {
+
+  @Override
+  protected void configurePreAgent() {
+    super.configurePreAgent()
+    injectSysConfig('dd.trace.java.lang.21.enabled=true', 'true')
+  }
+
   def "test Thread.Builder.OfVirtual - start()"() {
     setup:
     def threadBuilder = Thread.ofVirtual().name("builder - started")

dd-smoke-tests/concurrent/java-21/src/test/groovy/datadog/smoketest/concurrent/AbstractConcurrentTest.groovy

@@ -18,6 +18,7 @@ abstract class AbstractConcurrentTest extends AbstractSmokeTest {
     command.add(javaPath())
     command.addAll(defaultJavaProperties)
     command.add("-Ddd.trace.otel.enabled=true")
+    command.add("-Ddd.trace.java.lang.21.enabled=true")
     command.addAll(["-jar", jarPath])
     command.addAll(getTestArguments())