v3grpc: adds log interceptor unit tests

[redwrasse]

Adds unit tests for the log interceptorinterceptor.logUnaryRequestStats addressing #16823.

Code coverage has likely evolved since #16823 was opened. The unit tests in this PR try to test a few general properties of the interceptor.logUnaryRequestStats interceptor as well as the case of *pb.TxnResponse type response appearing to be missing coverage (coverage lines referenced in the description for #16823).

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @redwrasse. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.00%. Comparing base (deb9178) to head (ffc4bc9).

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/api/v3rpc/grpc.go	95.12% <100.00%> (ø)
server/etcdserver/api/v3rpc/interceptor.go	90.10% <100.00%> (+12.50%)	⬆️

... and 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19448      +/-   ##
==========================================
+ Coverage   68.90%   69.00%   +0.09%     
==========================================
  Files         420      420              
  Lines       35753    35753              
==========================================
+ Hits        24636    24670      +34     
+ Misses       9693     9660      -33     
+ Partials     1424     1423       -1     

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