HDFS-16029. Fix ArrayIndexOutOfBoundsException and divide-by-zero in InstrumentationService.Timer.getValues()

[balodesecurity]

Summary

• InstrumentationService.Timer.getValues() throws ArrayIndexOutOfBoundsException (index -1) and would divide by zero when called on a freshly constructed Timer before any Cron has been recorded (last == -1).
• Fix: return an all-zeros array immediately when last < 0, matching the suggestion in the JIRA ticket. This is the same defensive pattern used in Sampler.getRate() which guards against last == 0.

Test

• Added timerGetValuesBeforeAnyCron() to TestInstrumentationService — creates a fresh Timer(2), asserts getValues() returns zeros without exception, and also asserts toJSONString() does not throw.
• All 7 tests in TestInstrumentationService pass.

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	19m 6s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
+1 💚	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
			_ trunk Compile Tests _
+1 💚	mvninstall	41m 8s		trunk passed
+1 💚	compile	0m 56s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	compile	0m 55s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	checkstyle	0m 57s		trunk passed
+1 💚	mvnsite	1m 8s		trunk passed
+1 💚	javadoc	0m 57s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	0m 55s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	1m 16s		trunk passed
+1 💚	shadedclient	28m 22s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 24s		the patch passed
+1 💚	compile	0m 23s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javac	0m 23s		the patch passed
+1 💚	compile	0m 24s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	javac	0m 24s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 20s		the patch passed
+1 💚	mvnsite	0m 33s		the patch passed
+1 💚	javadoc	0m 23s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	0m 24s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	0m 55s		the patch passed
+1 💚	shadedclient	27m 11s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	5m 18s		hadoop-hdfs-httpfs in the patch passed.
+1 💚	asflicense	0m 36s		The patch does not generate ASF License warnings.
		136m 11s

Subsystem	Report/Notes
Docker	ClientAPI=1.54 ServerAPI=1.54 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8420/1/artifact/out/Dockerfile
GITHUB PR	#8420
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux ba87e5f739e5 5.15.0-173-generic #183-Ubuntu SMP Fri Mar 6 13:29:34 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / a6a4ddf
Default Java	Ubuntu-17.0.18+8-Ubuntu-124.04.1
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.10+7-Ubuntu-124.04 /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.18+8-Ubuntu-124.04.1
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8420/1/testReport/
Max. process+thread count	959 (vs. ulimit of 10000)
modules	C: hadoop-hdfs-project/hadoop-hdfs-httpfs U: hadoop-hdfs-project/hadoop-hdfs-httpfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8420/1/console
versions	git=2.43.0 maven=3.9.11 spotbugs=4.9.7
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[pan3793]

@balodesecurity, please strictly follow pull_request_template, especially, answer the question - whether the PR "Contains content generated by <tool>", for more details, read ASF Generative Tooling Guidance