RFR: 8358970: CounterMonitorMBean.getDerivedGaugeTimeStamp() JavaDoc incorrectly documents null

[Kevin Walls]

Simple correction where API docs state a return value may be "null", but in fact this is a lresult of type long, and may be zero.

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Commit messages:
 - 8358970: CounterMonitorMBean.getDerivedGaugeTimeStamp() JavaDoc incorrectly documents null

Changes: https://git.openjdk.org/jdk/pull/25694/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25694&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358970
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25694.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25694/head:pull/25694

PR: https://git.openjdk.org/jdk/pull/25694