How about considering add jmx support for dubbo

Prometheus may collect a lot of metrics .
For java , we often use jmx_exporter to collect metrics to expose to prometheus.
But now dubbo never provide some jmx bean.
Even in healthEndpoint, jmx_exporter cannot collect java type other than Number Boolean Date and String
ref to https://github.com/prometheus/jmx_exporter/issues/548
I think dubbo may expose own jmxbean to support this.
For example ,the threadpool size or other metrics may help our system more stable.

Any advice is welcome~