Cherry pick #7571 and #7579 to v1.66.x release branch

[purnesh42H]

Cherry-pick #7571 and #7579

RELEASE NOTES:

• mem: Fix minor server-side regression when pooling large (>1KiB) incoming request buffers
• experimental/stats: Remove dependency on testing package

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.83%. Comparing base (7185cf4) to head (057d663).
Report is 1 commits behind head on v1.66.x.

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.66.x    #7616      +/-   ##
===========================================
+ Coverage    81.74%   81.83%   +0.08%     
===========================================
  Files          361      361              
  Lines        27858    27858              
===========================================
+ Hits         22773    22798      +25     
+ Misses        3875     3861      -14     
+ Partials      1210     1199      -11     

Files with missing lines	Coverage Δ
experimental/stats/metricregistry.go	94.44% <100.00%> (-0.08%)	⬇️
mem/buffer_slice.go	96.20% <ø> (ø)
server.go	81.95% <100.00%> (+0.11%)	⬆️

... and 20 files with indirect coverage changes

[arjan-bal]

Can you keep this PR for a single cherry-pick and open another PR for the second one?

[purnesh42H]

Can you keep this PR for a single cherry-pick and open another PR for the second one?

Discussed offline. Followed this example #7019