Documentation/op-guide: add job="etcd" label matcher to Grafana dashboard

[gmiroshnykov]

Prometheus that scrapes etcd might also scrape other things, so we should specifically ask for etcd metrics, especially when dealing with generic metrics like process_resident_memory_bytes.

[codecov-io]

Codecov Report

Merging #9410 into master will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9410      +/-   ##
==========================================
+ Coverage   72.43%   72.59%   +0.15%     
==========================================
  Files         362      362              
  Lines       30793    30793              
==========================================
+ Hits        22305    22353      +48     
+ Misses       6859     6817      -42     
+ Partials     1629     1623       -6

Impacted Files	Coverage Δ
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
proxy/grpcproxy/watch.go	87.57% <0%> (-3.11%)	⬇️
integration/bridge.go	71.75% <0%> (-2.3%)	⬇️
lease/leasehttp/http.go	58.08% <0%> (-1.48%)	⬇️
etcdserver/cluster_util.go	76.64% <0%> (-1.46%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.72% <0%> (-1.4%)	⬇️
clientv3/op.go	72.56% <0%> (-1.33%)	⬇️
etcdserver/v3_server.go	77.89% <0%> (-1.29%)	⬇️
clientv3/watch.go	95.09% <0%> (-0.74%)	⬇️
... and 16 more

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[gyuho]

LGTM. Thanks for contribution!

ref. https://prometheus.io/docs/concepts/jobs_instances/