RFR: 8256156: JFR: Allow 'jfr' tool to show metadata without a recording

[Yang Yi]

On Tue, 29 Dec 2020 07:44:36 GMT, Yang Yi <github.com+5010047+kelthuzadx at openjdk.org> wrote:

> Hi all,
> 
> May I please have a review for this minor patch?
> 
> It simplifies the use of subcommand `metadata` of jfr tool. Recording 
> file is no longer mandatory, users can directly use `jfr metadata` to 
> output JDK builtin meta information. As JDK-8256156 mentioned, it also 
> supports events and categories filters:
> $ jfr metadata 
> $ jfr metadata recording.jfr # compatible
> $ jfr metadata --events jdk.ThreadStart,jdk.ThreadEnd
> $ jfr metadata --events jdk.ThreadStart,jdk.ThreadEnd recording.jfr
> $ jfr metadata --categories GC,Detailed
> $ jfr metadata --categories GC,Detailed recording.jfr

@egahlin  Hi Erik, I have updated my patch according to your review comments.

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PR: https://git.openjdk.java.net/jdk/pull/1904