Make sure we load core JMX using the deferred callback's inherited thread-context-classloader (TCCL)

This handles a situation where the app thread triggering JMX has a special thread-context-classloader
set that can see the custom JMX builder class. The callback thread inherits this TCCL so we just need
to make sure to load core JMX using this before starting JMXFetch (which needs a different TCCL set.)

Author: mcculls

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

@@ -1,6 +1,7 @@
 package datadog.trace.bootstrap;
 
 import java.lang.instrument.Instrumentation;
+import java.lang.management.ManagementFactory;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
@@ -281,6 +282,9 @@ private static synchronized void installDatadogTracer() {
   }
 
   private static synchronized void startJmx(final URL bootstrapURL) {
+    // load core JMX using the inherited thread-context-classloader
+    ManagementFactory.getPlatformMBeanServer();
+
     startJmxFetch(bootstrapURL);
 
     if (AGENT_CLASSLOADER == null) {