Expose jmx endpoint for sentinel #1814

Issue Description

Type: feature request

Describe what happened (or what feature you want)

jmx support for sentinel metrics

so we can use prometheus jmx_exporter to collect metric
also jmx is standard to manage our java bean.

Java Management Extensions (JMX) was introduced in J2SE 5.0 release. It provides an architecture to manage resources dynamically at runtime. JMX is used mostly in enterprise applications to make the system configurable or to get the state of application at any point of time.