HDFS-17933. [ARR] Add metrics to track async router queue size

[kokonguyen191]

Description of PR

Follow up to HDFS-17909. It's useful to have some metrics for async handler queue size to track the router pressure.

How was this patch tested?

Local deployment

[kokonguyen191]

Hi @ZanderXu @hfutatzhanghb @KeeProMise, please take a look when you are free, this is the follow up to #8448 to add some metrics for tracking

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	12m 37s		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 doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	41m 26s		trunk passed
+1 💚	compile	1m 7s		trunk passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	compile	1m 32s		trunk passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	checkstyle	1m 2s		trunk passed
+1 💚	mvnsite	1m 36s		trunk passed
+1 💚	javadoc	1m 9s		trunk passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	javadoc	1m 5s		trunk passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	spotbugs	2m 23s		trunk passed
+1 💚	shadedclient	30m 9s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 1s		the patch passed
+1 💚	compile	0m 39s		the patch passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	javac	0m 39s		the patch passed
+1 💚	compile	1m 0s		the patch passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	javac	1m 0s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 28s		the patch passed
+1 💚	mvnsite	1m 6s		the patch passed
+1 💚	javadoc	0m 33s		the patch passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
-1 ❌	javadoc	0m 34s	/results-javadoc-javadoc-hadoop-hdfs-project_hadoop-hdfs-rbf-jdkUbuntu-17.0.19+10-1-24.04.2-Ubuntu.txt	hadoop-hdfs-project_hadoop-hdfs-rbf-jdkUbuntu-17.0.19+10-1-24.04.2-Ubuntu with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu generated 2 new + 937 unchanged - 0 fixed = 939 total (was 937)
+1 💚	spotbugs	2m 4s		the patch passed
+1 💚	shadedclient	29m 18s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	44m 18s		hadoop-hdfs-rbf in the patch passed.
+1 💚	asflicense	0m 39s		The patch does not generate ASF License warnings.
		177m 23s

Subsystem	Report/Notes
Docker	ClientAPI=1.54 ServerAPI=1.54 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8539/1/artifact/out/Dockerfile
GITHUB PR	#8539
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 43f630fad7f0 5.15.0-176-generic #186-Ubuntu SMP Fri Mar 13 11:01:42 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / a26dbae
Default Java	Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.11+10-1-24.04.2-Ubuntu /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8539/1/testReport/
Max. process+thread count	3694 (vs. ulimit of 10000)
modules	C: hadoop-hdfs-project/hadoop-hdfs-rbf U: hadoop-hdfs-project/hadoop-hdfs-rbf
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8539/1/console
versions	git=2.43.0 maven=3.9.15 spotbugs=4.9.7
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[kokonguyen191]

Javadocs warning is from an unrelated file

[ZanderXu]

Make sense.

LGTM +1.