[jdk21u-dev] RFR: 8320687: sun.jvmstat.monitor.MonitoredHost.getMonitoredHost() throws unexpected exceptions when invoked concurrently

[Satyen Subramaniam]

Backporting JDK-8320687: sun.jvmstat.monitor.MonitoredHost.getMonitoredHost() throws unexpected exceptions when invoked concurrently. The fix proposes to guard the usage of the shared ServiceLoader instance through the monitoredHosts object monitor. Adds test. Ran GHA Sanity Checks, local Tier 1, 2, and 3, and new `test/jdk/sun/jvmstat/monitor/MonitoredVm/ConcurrentGetMonitoredHost.java` tests. Patch is clean.

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Commit messages:
 - Backport 81484d8c0520cf55ec58fc7b4c81880e69537674

Changes: https://git.openjdk.org/jdk21u-dev/pull/1714/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1714&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320687
  Stats: 103 lines in 2 files changed: 93 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1714.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1714/head:pull/1714

PR: https://git.openjdk.org/jdk21u-dev/pull/1714