docs: automate metrics list documentation

[gyuho]

Currently, need manual cluster set-up to find all available metrics, thus cannot be documented. Automatically document all available metrics, with embedded etcd for master branch or specified endpoints. This will make tracking metrics changes much easier.

Following PR should initialize grpc_server_msg_sent_total for all available RPCs (#8878). Right now, v3.1 and v3.2 only show the ones that have been requested.

# for latest master branch
etcd-dump-metrics > docs/metrics-latest

# download etcd v3.3 to ./bin
goreman start
etcd-dump-metrics -addr http://localhost:2379/metrics > docs/metrics-v3.3

# download etcd v3.2 to ./bin
goreman start
etcd-dump-metrics -addr http://localhost:2379/metrics > docs/metrics-v3.2

# download etcd v3.1 to ./bin
goreman start
etcd-dump-metrics -addr http://localhost:2379/metrics > docs/metrics-v3.1

Sample rendering (will be served via https://etcd.readthedocs.io/en/latest once merged)

For #9438.

[codecov-io]

Codecov Report

Merging #9950 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #9950      +/-   ##
=========================================
- Coverage   69.12%   69.1%   -0.02%     
=========================================
  Files         386     386              
  Lines       35784   35784              
=========================================
- Hits        24735   24730       -5     
- Misses       9232    9244      +12     
+ Partials     1817    1810       -7

Impacted Files	Coverage Δ
client/client.go	77.45% <0%> (-7.19%)	⬇️
clientv3/namespace/watch.go	72.72% <0%> (-6.07%)	⬇️
pkg/transport/listener.go	58.67% <0%> (-4.09%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
clientv3/maintenance.go	69.38% <0%> (-2.05%)	⬇️
pkg/adt/interval_tree.go	89.18% <0%> (-1.51%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
etcdserver/api/v3rpc/watch.go	74.18% <0%> (-0.99%)	⬇️
clientv3/client.go	70.96% <0%> (-0.88%)	⬇️
mvcc/watchable_store.go	82.8% <0%> (-0.71%)	⬇️
... and 13 more

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[wenjiaswe]

lgtm. Thanks @gyuho , very useful tool. I will start to play with it.

I will back port it to 3.3. and 3.2. I will check if it's OK to back port to 3.1 or if there is any modification needed.

[gyuho]

@wenjiaswe I am ok with backporting it but since http://etcd.readthedocs.io/en/latest/ is only serving on master branch for now, old metrics won't be shown for awhile. It would be helpful if you can regenerate those metrics using etcd-dump-metrics after me and @jpbetz finished today's release process.

[wenjiaswe]

OK, got it.