fix missing helper class definition in mongodb instrumentation

Author: na-ji

dd-java-agent/instrumentation/mongo/driver-3.6/src/main/java/datadog/trace/instrumentation/mongo/DefaultServerConnection36Instrumentation.java

@@ -32,6 +32,7 @@ public String instrumentedType() {
   @Override
   public String[] helperClassNames() {
     return new String[] {
+      "datadog.trace.core.propagation.W3CTraceParent",
       packageName + ".BsonScrubber",
       packageName + ".MongoCommentInjector",
       packageName + ".MongoDecorator",

dd-java-agent/instrumentation/mongo/driver-3.8/src/main/java/datadog/trace/instrumentation/mongo/DefaultServerConnection38Instrumentation.java

@@ -33,6 +33,7 @@ public String instrumentedType() {
   public String[] helperClassNames() {
     return new String[] {
       "datadog.trace.bootstrap.instrumentation.dbm.SharedDBCommenter",
+      "datadog.trace.core.propagation.W3CTraceParent",
       packageName + ".MongoDecorator",
       packageName + ".MongoCommentInjector",
       packageName + ".BsonScrubber",