clientv3: log warning in case of error sending request

[tedyu]

In watchGrpcStream#run, we should log the (non-nil) error when sending request.

[codecov-io]

Codecov Report

Merging #11452 into master will increase coverage by 0.11%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master   #11452      +/-   ##
==========================================
+ Coverage   64.11%   64.23%   +0.11%     
==========================================
  Files         403      403              
  Lines       38081    38085       +4     
==========================================
+ Hits        24416    24462      +46     
+ Misses      12037    11974      -63     
- Partials     1628     1649      +21

Impacted Files	Coverage Δ
clientv3/watch.go	90.81% <75%> (-0.77%)	⬇️
clientv3/balancer/utils.go	53.84% <0%> (-46.16%)	⬇️
auth/options.go	52.5% <0%> (-40%)	⬇️
pkg/transport/timeout_conn.go	60% <0%> (-20%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-7.38%)	⬇️
clientv3/leasing/util.go	91.66% <0%> (-6.67%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
clientv3/concurrency/mutex.go	62.16% <0%> (-5.41%)	⬇️
clientv3/leasing/cache.go	87.22% <0%> (-3.89%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
... and 19 more

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[tedyu]

@jingyih
Can you take a look ?

[jingyih]

Do we need to guard these log lines with if l.lg != nil {?

[tedyu]

@jingyih
Please take another look.

[jingyih]

LGTM. just one more nit.

[jingyih]

LGTM