HADOOP-19905. Implement HttpServer2 thread pool metrics

[magnuma3]

Description of PR

HADOOP-19905

HttpServer2Metrics (HADOOP-17133) exposes only StatisticsHandler
counters. It says nothing about the Jetty thread pool, so
administrators have no metric-level visibility into thread pool
saturation.
This patch adds gauges to HttpServer2Metrics:

• Pool: maxThreads, minThreads, threads, idleThreads, busyThreads, queueSize
• Per role (summed over connectors): acceptorThreads, selectorThreads
• Worker only (pool count minus acceptors and selectors):
  maxWorkerThreads, workerThreads, busyWorkerThreads
  The worker-only gauges are exposed separately because acceptor and
  selector threads sit in blocking accept()/select() loops and never go
  idle

How was this patch tested?

unit test and it has been running in production for over a year.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

AI Tooling

If an AI tool was used:

• The PR includes the phrase "Contains content generated by "
  where is the name of the AI tool used.
• My use of AI contributions follows the ASF legal policy
  https://www.apache.org/legal/generative-tooling.html

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	1m 1s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 1s		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	42m 20s		trunk passed
+1 💚	compile	16m 11s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	compile	16m 33s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	checkstyle	1m 30s		trunk passed
+1 💚	mvnsite	1m 57s		trunk passed
+1 💚	javadoc	1m 28s		trunk passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	1m 25s		trunk passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	3m 6s		trunk passed
+1 💚	shadedclient	30m 57s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 13s		the patch passed
+1 💚	compile	15m 32s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javac	15m 32s		the patch passed
+1 💚	compile	16m 31s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	javac	16m 31s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	1m 27s		the patch passed
+1 💚	mvnsite	1m 57s		the patch passed
+1 💚	javadoc	1m 27s		the patch passed with JDK Ubuntu-21.0.10+7-Ubuntu-124.04
+1 💚	javadoc	1m 24s		the patch passed with JDK Ubuntu-17.0.18+8-Ubuntu-124.04.1
+1 💚	spotbugs	3m 15s		the patch passed
+1 💚	shadedclient	30m 55s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	22m 39s		hadoop-common in the patch passed.
+1 💚	asflicense	1m 13s		The patch does not generate ASF License warnings.
		215m 24s

Subsystem	Report/Notes
Docker	ClientAPI=1.54 ServerAPI=1.54 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8520/1/artifact/out/Dockerfile
GITHUB PR	#8520
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux e604d24a746c 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 / 76ad71b
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-8520/1/testReport/
Max. process+thread count	1290 (vs. ulimit of 10000)
modules	C: hadoop-common-project/hadoop-common U: hadoop-common-project/hadoop-common
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8520/1/console
versions	git=2.43.0 maven=3.9.15 spotbugs=4.9.7
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