RFR: JDK-8273526: Extend the OSContainer API pids controller with pids.current

[Matthias Baesken]

https://bugs.openjdk.java.net/browse/JDK-8266490
extended the OSContainer API in order to also support the pids controller of cgroups. However only pids.max output was added with 8266490.
There is a second parameter pids.current , see https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#pid
that would be helpful too and can be added to the OSContainer API .
pids.current :
A read-only single value file which exists on all cgroups.
The number of processes currently in the cgroup and its descendants.

Best regards, Matthias

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Commit messages:
 - JDK-8273526

Changes: https://git.openjdk.java.net/jdk/pull/5437/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5437&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273526
  Stats: 104 lines in 15 files changed: 102 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5437.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5437/head:pull/5437

PR: https://git.openjdk.java.net/jdk/pull/5437