etcdserver/stats: fix stats data race.

[disksing]

Hi, the data race is recently reported from our CI. We are using etcd v3.2.14 as embed library.

WARNING: DATA RACE
Write at 0x00c425a78640 by goroutine 13:
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats.(*statsQueue).Insert()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats/queue.go:72 +0x3e3
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats.(*ServerStats).SendAppendReq()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats/server.go:120 +0x153
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/rafthttp.(*Transport).Send()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/rafthttp/transport.go:174 +0x343
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver.(*raftNode).start.func1()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/raft.go:214 +0xf4b
Previous read at 0x00c425a78640 by goroutine 99:
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats.(*statsQueue).Rate()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats/queue.go:43 +0x184
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats.(*ServerStats).JSON()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/stats/server.go:79 +0x122
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver.(*EtcdServer).SelfStats()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/server.go:1015 +0x58
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http.(*statsHandler).serveSelf()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http/client.go:301 +0x13b
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http.(*statsHandler).(github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http.serveSelf)-fm()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http/client.go:95 +0x5f
  net/http.HandlerFunc.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:1918 +0x51
  net/http.(*ServeMux).ServeHTTP()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:2254 +0xa2
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http.requestLogger.func1()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/api/v2http/http.go:72 +0x20f
  net/http.HandlerFunc.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:1918 +0x51
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/pkg/cors.(*CORSHandler).ServeHTTP()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/pkg/cors/cors.go:89 +0x16d
  net/http.(*ServeMux).ServeHTTP()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:2254 +0xa2
  net/http.serverHandler.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:2619 +0xbc
  net/http.(*conn).serve()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:1801 +0x83b
Goroutine 13 (running) created at:
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver.(*raftNode).start()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/raft.go:157 +0x5f
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver.(*EtcdServer).run()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/etcdserver/server.go:678 +0x58b
Goroutine 99 (finished) created at:
  net/http.(*Server).Serve()
      /home/travis/.gimme/versions/go1.9.4.linux.amd64/src/net/http/server.go:2720 +0x37c
  github.com/pingcap/pd/vendor/github.com/coreos/etcd/embed.(*serveCtx).serve.func2()
      /home/travis/gopath/src/github.com/pingcap/pd/vendor/github.com/coreos/etcd/embed/serve.go:116 +0x4c

[codecov-io]

Codecov Report

Merging #9562 into master will increase coverage by 0.1%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #9562     +/-   ##
=========================================
+ Coverage   72.23%   72.33%   +0.1%     
=========================================
  Files         368      368             
  Lines       31341    31341             
=========================================
+ Hits        22638    22670     +32     
+ Misses       7074     7031     -43     
- Partials     1629     1640     +11

Impacted Files	Coverage Δ
etcdserver/stats/server.go	81.03% <0%> (ø)	⬆️
etcdctl/ctlv3/command/get_command.go	46.59% <0%> (-30.69%)	⬇️
compactor/periodic.go	63.63% <0%> (-25.76%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-8.17%)	⬇️
etcdctl/ctlv3/command/lease_command.go	65.34% <0%> (-5.95%)	⬇️
pkg/transport/listener.go	60.54% <0%> (-3.25%)	⬇️
proxy/grpcproxy/register.go	80.55% <0%> (-2.78%)	⬇️
etcdserver/v3_server.go	78.37% <0%> (-2.55%)	⬇️
auth/simple_token.go	93.51% <0%> (-1.86%)	⬇️
pkg/adt/interval_tree.go	88.58% <0%> (-1.81%)	⬇️
... and 14 more

---

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[gyuho]

switch line 79 and 80?

[disksing]

@gyuho Fixed.

[gyuho]

lgtm thanks!