etcdctl: support query count only of specified prefix

[mlmhl]

Sometimes we only want to know how many objects under a specified prefix, instead of extracting the whole keys or values.

[codecov-io]

Codecov Report

Merging #11743 into master will decrease coverage by 0.05%.
The diff coverage is 12.50%.

@@            Coverage Diff             @@
##           master   #11743      +/-   ##
==========================================
- Coverage   66.12%   66.06%   -0.06%     
==========================================
  Files         403      403              
  Lines       36792    36800       +8     
==========================================
- Hits        24327    24313      -14     
- Misses      10967    10973       +6     
- Partials     1498     1514      +16     

Impacted Files	Coverage Δ
etcdctl/ctlv3/command/get_command.go	16.66% <12.50%> (-0.38%)	⬇️
client/client.go	44.11% <0.00%> (-22.55%)	⬇️
pkg/transport/timeout_conn.go	60.00% <0.00%> (-20.00%)	⬇️
client/keys.go	76.88% <0.00%> (-14.58%)	⬇️
proxy/grpcproxy/register.go	72.50% <0.00%> (-10.00%)	⬇️
pkg/netutil/netutil.go	61.47% <0.00%> (-7.38%)	⬇️
pkg/tlsutil/tlsutil.go	86.20% <0.00%> (-6.90%)	⬇️
clientv3/leasing/util.go	91.66% <0.00%> (-6.67%)	⬇️
clientv3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
etcdserver/api/v3rpc/lease.go	77.21% <0.00%> (-5.07%)	⬇️
... and 24 more

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[mlmhl]

@gyuho @mitake PTAL :)

[xiang90]

The change LGTM.

@mlmhl Can you add this new feature to the changelog? Thanks

[mlmhl]

@xiang90 Of cause, I submitted a PR to update the CHANGELOG, PTAL :).

[mitake]

LGTM, thanks!