snapshot: use structured logger

[gyuho]

In the process of removing capnslog

Address #9438

[codecov-io]

Codecov Report

Merging #9538 into master will increase coverage by <.01%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master    #9538      +/-   ##
==========================================
+ Coverage   72.39%   72.39%   +<.01%     
==========================================
  Files         367      367              
  Lines       31267    31277      +10     
==========================================
+ Hits        22635    22643       +8     
- Misses       7000     7003       +3     
+ Partials     1632     1631       -1

Impacted Files	Coverage Δ
etcdctl/ctlv3/command/snapshot_command.go	67% <50%> (ø)	⬆️
snapshot/v3_snapshot.go	63.71% <88.88%> (+0.75%)	⬆️
etcdctl/ctlv2/command/role_commands.go	39.88% <0%> (-17.27%)	⬇️
auth/simple_token.go	87.03% <0%> (-6.49%)	⬇️
pkg/logutil/discard_logger.go	6.25% <0%> (-6.25%)	⬇️
etcdserver/api/v3rpc/watch.go	76.59% <0%> (-4.69%)	⬇️
etcdserver/api/v3rpc/lease.go	77.02% <0%> (-4.06%)	⬇️
clientv3/concurrency/election.go	79.52% <0%> (-2.37%)	⬇️
raft/progress.go	94.17% <0%> (-1.95%)	⬇️
clientv3/balancer/grpc1.7-health.go	85.17% <0%> (-1.46%)	⬇️
... and 15 more

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