Extend jcmd to java application level

[Denghui Dong]

Hi team,

The `jcmd` command can be used to call some built-in diagnostic commands in vm.
Can we consider extending it to the java layer like perf data, so that Java developers can
customize their diagnostic commands and then call them through `jcmd`?

One application scenario I can think of for this extension is that some statistical information
may be collected in a java application. Triggering the output of this statistical information through
the `jcmd` command seems to me relative to other mechanisms that trigger output (such as through
an HTTP service, or periodic Printing) is more convenient.

Any input is appreciated.

Thanks,
Denghui Dong
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