Adding workaround for Google App Engine - where some JRE classes are blacklisted

Hello,

I've tried to use metrics-servlets in a PaaS environments, where java.lang.management classes a re blacklisted to secure the running environment. I could possibly map each servlet in web.xml, but I like more to have a single servlet definition to AdminServlet. Therefore I added try catch around java.lang.managament class usage so in environments that like Google App Engine restricts them, we see "Sorry thread dump is not supported" message.