RFR: 8256156: JFR: Allow 'jfr' tool to show metadata without a recording [v8]

[Yang Yi]

> 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

Yang Yi has updated the pull request incrementally with one additional commit since the last revision:

  separate code into another method

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1904/files
  - new: https://git.openjdk.java.net/jdk/pull/1904/files/586a1824..f4faa40a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1904&range=07
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1904&range=06-07

  Stats: 34 lines in 1 file changed: 17 ins; 15 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1904.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1904/head:pull/1904

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