Reuse grpcOverheadBytes calculation by creating a function for that

[serathius]

cc @ahrtr @jmhbnz @fuweid

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.74%. Comparing base (6fa7342) to head (c77ea65).
Report is 2 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
server/config/config.go	80.23% <100.00%> (+0.47%)	⬆️
server/etcdserver/api/v3rpc/grpc.go	100.00% <100.00%> (ø)
server/etcdserver/api/v3rpc/watch.go	84.06% <100.00%> (+1.25%)	⬆️

... and 30 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18991      +/-   ##
==========================================
+ Coverage   68.53%   68.74%   +0.21%     
==========================================
  Files         420      420              
  Lines       35619    35621       +2     
==========================================
+ Hits        24410    24489      +79     
+ Misses       9777     9702      -75     
+ Partials     1432     1430       -2     

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[fuweid]

LGTM

[serathius]

/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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