RFR: 8357828: Add a timestamp to jcmd diagnostic commands

[Alan Bateman]

On Thu, 18 Sep 2025 14:57:44 GMT, Ivan Bereziuk <duke at openjdk.org> wrote:

> `jcmd` provides great diagnostics but many commands lack a timestamp in their output.
> Adding a timestamp to the output of some would add value for those debugging JVM data.
> 
> Some diagnostic commands already provide timestamps. For example `Thread.print` already prints one of "yyyy-MM-dd HH:mm:ss" format.
> 
> Adding timestamps to all diagnostic `jcmd` commands with a non-throw-away STDOUT.
> 
> The exceptions are:
> * `VM.uptime` - command run with `-date` argument will also print a timestamp;
> * `VM.system_properties` - already lists timestamp
> * `Thread.dump_to_file` - the content dumped to file already has a timestamp;
> * `VM.version`

It might be useful for @larry-cable to comment on this too. I think he's exploring some of these command serving up JSON rather than plain text. Any "infrastructure" that print a plain text timestamp could mess that up.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/27368#issuecomment-3323199390