*: upgrade to grpclog.LoggerV2

[gyuho]

grpclog.Logger has been deprecated.

[heyitsanthony]

lgtm thanks

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@80aa810). Click here to learn what that means.
The diff coverage is 85.3%.

@@            Coverage Diff            @@
##             master    #8533   +/-   ##
=========================================
  Coverage          ?   76.21%           
=========================================
  Files             ?      359           
  Lines             ?    29740           
  Branches          ?        0           
=========================================
  Hits              ?    22667           
  Misses            ?     5501           
  Partials          ?     1572

Impacted Files	Coverage Δ
clientv3/watch.go	96.28% <0%> (ø)
auth/simple_token.go	93.51% <100%> (ø)
clientv3/auth.go	95.65% <100%> (ø)
clientv3/ordering/kv.go	85.29% <100%> (ø)
clientv3/concurrency/election.go	81.88% <100%> (ø)
clientv3/ready_wait.go	100% <100%> (ø)
client/keys.go	91.45% <100%> (ø)
client/client.go	84.64% <100%> (ø)
clientv3/kv.go	96.77% <100%> (ø)
clientv3/txn.go	100% <100%> (ø)
... and 14 more

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