Support OpenTelemetry extension helpers

dd-java-agent/agent-otel/otel-tooling/src/main/java/datadog/opentelemetry/tooling/OtelInstrumentationMapper.java

@@ -23,9 +23,7 @@ public final class OtelInstrumentationMapper extends ClassRemapper {
           Arrays.asList("io/opentelemetry/javaagent/tooling/muzzle/InstrumentationModuleMuzzle"));
 
   private static final Set<String> UNSUPPORTED_METHODS =
-      new HashSet<>(
-          Arrays.asList(
-              "getMuzzleReferences", "getMuzzleHelperClassNames", "registerMuzzleVirtualFields"));
+      new HashSet<>(Arrays.asList("getMuzzleReferences", "registerMuzzleVirtualFields"));
 
   public OtelInstrumentationMapper(ClassVisitor classVisitor) {
     super(classVisitor, Renamer.INSTANCE);

dd-java-agent/agent-otel/otel-tooling/src/main/java/datadog/opentelemetry/tooling/OtelInstrumenterModule.java

@@ -2,6 +2,9 @@
 
 import datadog.trace.agent.tooling.InstrumenterModule;
 import datadog.trace.api.InstrumenterConfig;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
 /**
  * Replaces OpenTelemetry's {@code InstrumentationModule} when mapping extensions.
@@ -30,4 +33,33 @@ private static String[] namespace(String[] names) {
     }
     return namespaced;
   }
+
+  @Override
+  public String[] helperClassNames() {
+    List<String> helperClassNames;
+    List<String> additionalClassNames = getAdditionalHelperClassNames();
+    if (additionalClassNames.isEmpty()) {
+      // no additional names, just use those captured at build time
+      helperClassNames = getMuzzleHelperClassNames();
+    } else {
+      // combine names captured at build time with additional names
+      helperClassNames = new ArrayList<>(getMuzzleHelperClassNames());
+      helperClassNames.addAll(additionalClassNames);
+    }
+    if (helperClassNames.isEmpty()) {
+      return NO_HELPERS;
+    } else {
+      return helperClassNames.toArray(new String[0]);
+    }
+  }
+
+  /** This method is generated by OpenTelemetry's muzzle plugin at build time. */
+  public List<String> getMuzzleHelperClassNames() {
+    return Collections.emptyList();
+  }
+
+  /** Extensions override this to supply names not covered by the muzzle plugin. */
+  public List<String> getAdditionalHelperClassNames() {
+    return Collections.emptyList();
+  }
 }

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/InstrumenterModule.java

@@ -54,6 +54,8 @@ public enum TargetSystem {
 
   private static final Logger log = LoggerFactory.getLogger(InstrumenterModule.class);
 
+  protected static final String[] NO_HELPERS = {};
+
   private final List<String> instrumentationNames;
   private final String instrumentationPrimaryName;
   private final boolean enabled;
@@ -106,7 +108,7 @@ public static ReferenceMatcher loadStaticMuzzleReferences(
 
   /** @return Class names of helpers to inject into the user's classloader */
   public String[] helperClassNames() {
-    return new String[0];
+    return NO_HELPERS;
   }
 
   /**