etcdctl/v3: enable 'require-leader' for 'watch' command

[gyuho]

Address #7941 (comment).

[xiang90]

i think the require leader should probably be enabled for ctl tool.

[xiang90]

so no need the flag.

[gyuho]

@xiang90

should probably be enabled for ctl tool.

You mean by default, enabled for watch?

[xiang90]

yes, enable by default for watch. etcdctl users would expect the command to be always interactive. it is probably the best option to simply enable require leader and do not provide that as a flag.

[gyuho]

@xiang90 PTAL.

[xiang90]

lgtm

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@ed92420). Click here to learn what that means.
The diff coverage is 85.56%.

@@            Coverage Diff            @@
##             master    #8672   +/-   ##
=========================================
  Coverage          ?   76.01%           
=========================================
  Files             ?      360           
  Lines             ?    29643           
  Branches          ?        0           
=========================================
  Hits              ?    22533           
  Misses            ?     5544           
  Partials          ?     1566

Impacted Files	Coverage Δ
clientv3/client.go	84.32% <ø> (ø)
etcdserver/api/v3rpc/rpctypes/error.go	90.47% <ø> (ø)
clientv3/watch.go	94.8% <ø> (ø)
etcdctl/ctlv3/command/make_mirror_command.go	15.11% <0%> (ø)
etcdctl/ctlv3/command/watch_command.go	15.15% <0%> (ø)
etcdctl/ctlv3/command/check.go	14.14% <0%> (ø)
clientv3/txn.go	100% <100%> (ø)
clientv3/auth.go	95.65% <100%> (ø)
clientv3/ready_wait.go	100% <100%> (ø)
clientv3/kv.go	96.77% <100%> (ø)
... and 12 more

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