Add per-operation 404 counters

[nicolaslopezbravo]

Summary

Add per-operation NotFoundCount counters so object-GET, HEAD, and LIST 404 rates can be monitored independently. Count each request once using its recorded response status; preserve existing counters.

Durability: storage operations, response handling, and persisted formats remain unchanged. Copilot assisted implementation and test coverage.

Testing Done

• Local code review completed.
• 49 targeted cases passed: request metrics/tracker (9), Netty response channel (38), and frontend routing/variant isolation (2).

JAVA_HOME=$(/usr/libexec/java_home -v 17) ./gradlew --no-daemon --max-workers=2 -PdisableShipkit \
  :ambry-api:jar :ambry-api:test \
  --tests com.github.ambry.rest.RestRequestMetricsTest \
  --tests com.github.ambry.rest.RestRequestMetricsTrackerTest \
  :ambry-rest:test --tests com.github.ambry.rest.NettyResponseChannelTest \
  :ambry-frontend:test \
  --tests com.github.ambry.frontend.FrontendRestRequestServiceTest.testNotFoundMetricsFollowS3Routing \
  --tests com.github.ambry.frontend.FrontendRestRequestServiceTest.testNotFoundMetricsIsolateTransportAndEncryptionVariants

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.72%. Comparing base (52ba813) to head (fbddb36).
⚠️ Report is 420 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3299       +/-   ##
=============================================
- Coverage     64.24%   50.72%   -13.53%     
+ Complexity    10398     8714     -1684     
=============================================
  Files           840      939       +99     
  Lines         71755    80794     +9039     
  Branches       8611     9749     +1138     
=============================================
- Hits          46099    40980     -5119     
- Misses        23004    36417    +13413     
- Partials       2652     3397      +745     

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