docs,tools/etcd-dump-metrics: automate metrics documentation

[gyuho]

Address #9438.

/cc @wenjiaswe

[codecov-io]

Codecov Report

Merging #9984 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9984      +/-   ##
==========================================
+ Coverage   69.34%   69.35%   +0.01%     
==========================================
  Files         386      386              
  Lines       35914    35914              
==========================================
+ Hits        24905    24909       +4     
+ Misses       9212     9210       -2     
+ Partials     1797     1795       -2

Impacted Files	Coverage Δ
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
clientv3/namespace/watch.go	72.72% <0%> (-6.07%)	⬇️
pkg/logutil/zap_grpc.go	47.61% <0%> (-4.77%)	⬇️
pkg/transport/listener.go	58.67% <0%> (-4.09%)	⬇️
clientv3/concurrency/election.go	79.52% <0%> (-2.37%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.48% <0%> (-1.35%)	⬇️
pkg/adt/interval_tree.go	90.39% <0%> (-0.91%)	⬇️
etcdserver/api/rafthttp/stream.go	79.39% <0%> (-0.43%)	⬇️
clientv3/watch.go	91.5% <0%> (-0.43%)	⬇️
pkg/proxy/server.go	59.86% <0%> (-0.34%)	⬇️
... and 13 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 93be31d...902893f. Read the comment docs.

[wenjiaswe]

@gyuho I am sorry I forgot to follow up with #9950. I will do this after future releases. Thanks!

[gyuho]

@wenjiaswe That would be great. Thanks!