etcdserver: add "etcd_server_go_version" metric

[gyuho]

Currently, one has to look at server logs manually,
to see what Go version was used to build etcd server.

# TYPE etcd_server_go_version gauge
etcd_server_go_version{server_go_version="go1.10.3"} 1

@wenjiaswe

[xiang90]

lgtm

[codecov-io]

Codecov Report

Merging #9957 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9957      +/-   ##
==========================================
+ Coverage   69.02%   69.03%   +<.01%     
==========================================
  Files         386      386              
  Lines       35788    35792       +4     
==========================================
+ Hits        24704    24710       +6     
+ Misses       9282     9271      -11     
- Partials     1802     1811       +9

Impacted Files	Coverage Δ
etcdserver/metrics.go	64.58% <100%> (+3.21%)	⬆️
etcdserver/util.go	81.25% <0%> (-13.75%)	⬇️
etcdserver/api/v3rpc/lease.go	65.9% <0%> (-11.37%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-6.56%)	⬇️
clientv3/namespace/watch.go	72.72% <0%> (-6.07%)	⬇️
pkg/logutil/zap_grpc.go	47.61% <0%> (-4.77%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
proxy/httpproxy/director.go	80% <0%> (-2.86%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (-2.78%)	⬇️
etcdserver/api/v3rpc/watch.go	74.18% <0%> (-2.29%)	⬇️
... and 14 more

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[wenjiaswe]

LGTM
Will this be tested and documented?

[gyuho]

@wenjiaswe Yes, I will document in changelogs, but don't think it needs be tested :)