rafthttp: add peer stream metrics

[gyuho]

No description provided.

[gyuho]

Just leave this as future reference in case someone asks.
Currently we don't have any use case of it. If anyone needs to
count the connection between peers, this might be useful.
Right now, only way to find out is just looking at the logs.

e.g. with this patch, in 5-node cluster, etcd_network_peer_active_total
should be 20 (4 peers * 5 connections each).

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@04fc57a). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #7682   +/-   ##
=========================================
  Coverage          ?   75.92%           
=========================================
  Files             ?      331           
  Lines             ?    26088           
  Branches          ?        0           
=========================================
  Hits              ?    19807           
  Misses            ?     4850           
  Partials          ?     1431

Impacted Files	Coverage Δ
rafthttp/peer_status.go	87.09% <100%> (ø)
rafthttp/metrics.go	100% <100%> (ø)

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[xiang90]

when we remove the peer from the network, do we call inactive? if not, we probably also need to dec the counter there.

[xiang90]

e.g. with this patch, in 5-node cluster, etcd_network_peer_active_total
should be 20 (4 peers * 5 connections each).

we'd either make this 4 or change the metrics name to active_peer_connection with peer name as label.