Style guide: clarify final for javaagent modules (option 2 - use final in javaagent modules)

[trask]

#17344 (comment)

and applied to javaagent source

rg -l -0 --glob 'instrumentation/**/javaagent/src/main/java/**/*.java' '^public class ' | xargs -0 sed -i 's/^public class /public final class /'
./gradlew spotlessApply
git commit -a -m "Apply to javaagent files"

Option 1:

• Style guide: clarify final for javaagent modules (option 1 - don't use final in javaagent modules) #17417