RFR: 8340363: Tag-specific default decorators for UnifiedLogging [v3]

[Antón Seoane]

> Hi all,
> 
> Currently, the Unified Logging framework defaults to three decorators (uptime, level, tags) whenever the user does not specify otherwise through `-Xlog`. This can result in cumbersome input whenever a specific user that relies on a particular tag(s) has some predefined needs. For example, C2 developers rarely need decorations, and having to manually specify this every time results inconvenient.
> 
> To address this, this PR enables the possibility of adding tag-specific default decorators to UL. These defaults are in no way overriding user input -- they will only act whenever `-Xlog` has no decorators supplied and there is a positive match with the pre-specified defaults. Such a match is based on the following:
> 
> - Inclusion: if `-Xlog:jit+compilation` is provided, a default for `jit` may be applied.
> - Specificity: if, for the above line, there is a more specific default for `jit+compilation` the latter shall be applied. Upon equal specificity cases, both defaults will be applied.
> - Additionally, defaults may target a specific log level.
> 
> Decorators are also associated with an output file, so an output device may only have one set of decorators. For this reason, if different `LogSelection`s trigger defaults, none is to be applied.
> 
> In summary, these defaults may be seen as a "tailored" or "flavoured" version of the existing "uptime-level-tags" current defaults.
> 
> Please consider this PR, and thanks!

Antón Seoane has updated the pull request incrementally with one additional commit since the last revision:

  Initialization of _decorators field in logDecorators

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20988/files
  - new: https://git.openjdk.org/jdk/pull/20988/files/af0b27be..ee24f637

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20988&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20988&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20988.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20988/head:pull/20988

PR: https://git.openjdk.org/jdk/pull/20988