Classify DELETE latency by observed dispatch and repair

[nicolaslopezbravo]

Keep request-level TTFB cohorts additive so routing costs can be compared without changing combined metrics, completion, or durability behavior.

Summary

Testing Done

[codecov-commenter]

Codecov Report

❌ Patch coverage is 9.83607% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.00%. Comparing base (52ba813) to head (fe5a7bb).
⚠️ Report is 421 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/com/github/ambry/rest/DeleteRequestMetrics.java	0.00%	31 Missing ⚠️
...m/github/ambry/rest/RestRequestMetricsTracker.java	0.00%	6 Missing ⚠️
...ava/com/github/ambry/frontend/FrontendMetrics.java	0.00%	4 Missing ⚠️
...github/ambry/frontend/s3/S3BatchDeleteHandler.java	0.00%	4 Missing ⚠️
.../java/com/github/ambry/router/DeleteOperation.java	33.33%	3 Missing and 1 partial ⚠️
...a/com/github/ambry/frontend/DeleteBlobHandler.java	0.00%	3 Missing ⚠️
...ava/com/github/ambry/router/NonBlockingRouter.java	40.00%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (52ba813) and HEAD (fe5a7bb). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (52ba813)	HEAD (fe5a7bb)
	3	2

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3302       +/-   ##
=============================================
- Coverage     64.24%   38.00%   -26.25%     
+ Complexity    10398     6479     -3919     
=============================================
  Files           840      940      +100     
  Lines         71755    80850     +9095     
  Branches       8611     9758     +1147     
=============================================
- Hits          46099    30724    -15375     
- Misses        23004    47643    +24639     
+ Partials       2652     2483      -169     

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