RFR: 8254001: [Metrics] Enhance parsing of cgroup interface files for version detection [v5]

[Severin Gehwolf]

> This is an enhancement which solves two issues:
> 
> 1. Multiple reads of relevant cgroup interface files. Now interface files are only read once per file (just like Hotspot).
> 2. Proxies creation of the impl specific subsystem via `determineType()` as before, but now reads all relevant interface files: `/proc/cgroups`, `/proc/self/mountinfo` and `/proc/self/cgroup`. Once read it passes the parsed information to the impl specific subsystem classes for instantiation. This allows for more flexibility of testing as interface files can be mocked and, thus, more cases can be tested that way without having access to these specific systems. For example, proper regression tests for JDK-8217766 and JDK-8253435 have been added now with this in place.
> 
> * [x] Tested on Linux x86_64 on cgroups v1 and cgroups v2. Container tests pass.

Severin Gehwolf has updated the pull request incrementally with one additional commit since the last revision:

  Add comment to parsing logic of /proc/self/cgroup

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1393/files
  - new: https://git.openjdk.java.net/jdk/pull/1393/files/9a2d6a20..078e16db

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1393&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1393&range=03-04

  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1393.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1393/head:pull/1393

PR: https://git.openjdk.java.net/jdk/pull/1393