HDFS-17689. RBF:Add some docs for RouterRpcServer#getRPCMetrics method.

[hfutatzhanghb]

Description of PR

Add some docs for RouterRpcServer#getRPCMetrics method because this method may return null if dfs.federation.router.metrics.enable is false.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 56s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 1s		codespell was not available.
+0 🆗	detsecrets	0m 1s		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	35m 10s		trunk passed
+1 💚	compile	0m 41s		trunk passed with JDK Ubuntu-11.0.25+9-post-Ubuntu-1ubuntu120.04
+1 💚	compile	0m 38s		trunk passed with JDK Private Build-1.8.0_432-8u432-gaus1-0ubuntu220.04-ga
+1 💚	checkstyle	0m 32s		trunk passed
+1 💚	mvnsite	0m 42s		trunk passed
+1 💚	javadoc	0m 44s		trunk passed with JDK Ubuntu-11.0.25+9-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	0m 33s		trunk passed with JDK Private Build-1.8.0_432-8u432-gaus1-0ubuntu220.04-ga
+1 💚	spotbugs	1m 25s		trunk passed
+1 💚	shadedclient	33m 8s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 30s		the patch passed
+1 💚	compile	0m 31s		the patch passed with JDK Ubuntu-11.0.25+9-post-Ubuntu-1ubuntu120.04
+1 💚	javac	0m 31s		the patch passed
+1 💚	compile	0m 28s		the patch passed with JDK Private Build-1.8.0_432-8u432-gaus1-0ubuntu220.04-ga
+1 💚	javac	0m 28s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 19s		the patch passed
+1 💚	mvnsite	0m 33s		the patch passed
+1 💚	javadoc	0m 28s		the patch passed with JDK Ubuntu-11.0.25+9-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	0m 24s		the patch passed with JDK Private Build-1.8.0_432-8u432-gaus1-0ubuntu220.04-ga
+1 💚	spotbugs	1m 19s		the patch passed
+1 💚	shadedclient	33m 4s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	35m 25s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs-rbf.txt	hadoop-hdfs-rbf in the patch passed.
+1 💚	asflicense	0m 36s		The patch does not generate ASF License warnings.
		149m 43s

Reason	Tests
Failed junit tests	hadoop.hdfs.server.federation.router.TestRouterClientRejectOverload

Subsystem	Report/Notes
Docker	ClientAPI=1.47 ServerAPI=1.47 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7236/1/artifact/out/Dockerfile
GITHUB PR	#7236
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux ecb1705639b7 5.15.0-124-generic #134-Ubuntu SMP Fri Sep 27 20:20:17 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 8f22813
Default Java	Private Build-1.8.0_432-8u432-gaus1-0ubuntu220.04-ga
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.25+9-post-Ubuntu-1ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_432-8u432-gaus1-0ubuntu220.04-ga
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7236/1/testReport/
Max. process+thread count	3200 (vs. ulimit of 5500)
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-7236/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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