RFR: 8320687: sun.jvmstat.monitor.MonitoredHost.getMonitoredHost() throws unexpected exceptions when invoked concurrently [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to fix the issue noted in https://bugs.openjdk.org/browse/JDK-8320687?
> 
> As noted in the issue, the `sun.jvmstat.monitor.MonitoredHost.getMonitoredHost()` uses a shared instance of `java.util.ServiceLoader` to load `MonitoredHostService` services. The `ServiceLoader` class javadoc explicitly notes that it isn't thread safe. The issue at hand is caused to due using an instance of `ServiceLoader` concurrently by multiple threads.
> 
> The fix proposes to guard the usage of the shared `ServiceLoader` instance through the `monitoredHosts` object monitor. We already use that monitor when dealing with the internal cache which is populated after loading the relevant `MonitoredHostService`(s).
> 
> A new jtreg test has been introduced which always reproduces the issue without the source changes and passes with this fix.
> 
> tier1, tier2, tier3 and svc_tools tests have been run with this change and all passed.

Jaikiran Pai has updated the pull request incrementally with two additional commits since the last revision:

 - Alan's review suggestion - rename GetMonitoredHost to ConcurrentGetMonitoredHost
 - fix code comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16805/files
  - new: https://git.openjdk.org/jdk/pull/16805/files/acc9e3dd..78e5cd7b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16805&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16805&range=00-01

  Stats: 7 lines in 2 files changed: 2 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16805.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16805/head:pull/16805

PR: https://git.openjdk.org/jdk/pull/16805