raft: avoid allocation of Raft entry due to logging

[nvanbenschoten]

raftpb.Entry.String takes a pointer receiver, so calling it
on a loop variable was causing the variable to escape. Removing
the .String() call was enough to avoid the allocation, but
this also avoids a memory copy and prevents similar bugs.

This was responsible for 11.63% of total allocations in an
experiment with https://github.com/nvanbenschoten/raft-toy.

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

[nvanbenschoten]

cc. @bdarnell

[codecov-io]

Codecov Report

Merging #10680 into master will decrease coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10680      +/-   ##
==========================================
- Coverage   71.63%   71.46%   -0.18%     
==========================================
  Files         394      394              
  Lines       36658    36659       +1     
==========================================
- Hits        26261    26197      -64     
- Misses       8564     8619      +55     
- Partials     1833     1843      +10

Impacted Files	Coverage Δ
raft/raft.go	91.55% <100%> (-0.64%)	⬇️
pkg/fileutil/purge.go	65.9% <0%> (-6.82%)	⬇️
etcdserver/apply.go	82.97% <0%> (-5.9%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
etcdserver/util.go	95% <0%> (-3.75%)	⬇️
proxy/grpcproxy/watch.go	89.75% <0%> (-3.02%)	⬇️
clientv3/balancer/balancer.go	84.25% <0%> (-2.37%)	⬇️
lease/leasehttp/http.go	64.23% <0%> (-1.46%)	⬇️
pkg/schedule/schedule.go	81.69% <0%> (-1.41%)	⬇️
... and 22 more

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[xiang90]

lgtm