HADOOP-17113, HADOOP-17158. Adding ReadAhead Counters and fixing ITestAbfsInputStreamStatistics#testReadAheadCounters

[mehakmeet]

Backport for both HADOOP-17113(#2154) and HADOOP-17158(#2272)
Tested by: mvn -T 1C -Dparallel-tests=abfs clean verify
Region: East US

[INFO] Results:
[INFO]
[INFO] Tests run: 87, Failures: 0, Errors: 0, Skipped: 0

[INFO] Results:
[INFO]
[WARNING] Tests run: 451, Failures: 0, Errors: 0, Skipped: 71

[INFO] Results:
[INFO]
[WARNING] Tests run: 206, Failures: 0, Errors: 0, Skipped: 29

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	31m 4s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+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.
		_ branch-3.3 Compile Tests _
+1 💚	mvninstall	28m 33s	branch-3.3 passed
+1 💚	compile	0m 32s	branch-3.3 passed
+1 💚	checkstyle	0m 27s	branch-3.3 passed
+1 💚	mvnsite	0m 37s	branch-3.3 passed
+1 💚	shadedclient	15m 4s	branch has no errors when building and testing our client artifacts.
+1 💚	javadoc	0m 27s	branch-3.3 passed
+0 🆗	spotbugs	0m 55s	Used deprecated FindBugs config; considering switching to SpotBugs.
+1 💚	findbugs	0m 52s	branch-3.3 passed
		_ Patch Compile Tests _
+1 💚	mvninstall	0m 30s	the patch passed
+1 💚	compile	0m 24s	the patch passed
+1 💚	javac	0m 24s	the patch passed
+1 💚	checkstyle	0m 16s	the patch passed
+1 💚	mvnsite	0m 27s	the patch passed
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	shadedclient	13m 51s	patch has no errors when building and testing our client artifacts.
+1 💚	javadoc	0m 24s	the patch passed
+1 💚	findbugs	0m 55s	the patch passed
		_ Other Tests _
+1 💚	unit	1m 23s	hadoop-azure in the patch passed.
+1 💚	asflicense	0m 33s	The patch does not generate ASF License warnings.
		98m 8s

Subsystem	Report/Notes
Docker	ClientAPI=1.40 ServerAPI=1.40 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2282/1/artifact/out/Dockerfile
GITHUB PR	#2282
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient findbugs checkstyle
uname	Linux 54bd4a666c1f 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	branch-3.3 / 5710005
Default Java	Private Build-1.8.0_265-8u265-b01-0ubuntu2~16.04-b01
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2282/1/testReport/
Max. process+thread count	418 (vs. ulimit of 5500)
modules	C: hadoop-tools/hadoop-azure U: hadoop-tools/hadoop-azure
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2282/1/console
versions	git=2.7.4 maven=3.3.9 findbugs=3.1.0-RC1
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[steveloughran]

merged to branch-3.3. Thanks!