[metrics] Rework on network metrics.

[jolestar]

重构 block relay metrics

1. 将多个 txns_filled metris 合并成 Vec，并通过 Counter 替代 Guage。
2. block_txns_count metrics 删除，因为从 block_txns_count 的 Guage 无法得出一个有效的监控信息。

[codecov]

Codecov Report

Merging #2967 (16fedce) into master (7ef1c62) will increase coverage by 0.44%.
The diff coverage is 27.42%.

@@            Coverage Diff             @@
##           master    #2967      +/-   ##
==========================================
+ Coverage   33.03%   33.46%   +0.44%     
==========================================
  Files         516      516              
  Lines       45269    45292      +23     
  Branches    20623    20635      +12     
==========================================
+ Hits        14948    15154     +206     
+ Misses      15962    15954       -8     
+ Partials    14359    14184     -175     

Flag	Coverage Δ
unittests	33.46% <27.42%> (+0.44%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/peer-watcher/src/lib.rs	100.00% <ø> (ø)
network/src/service.rs	30.84% <0.00%> (+0.16%)	⬆️
network/src/worker.rs	33.88% <0.00%> (ø)
block-relayer/src/block_relayer.rs	25.14% <27.28%> (+1.44%)	⬆️
block-relayer/src/metrics.rs	33.34% <29.63%> (-5.12%)	⬇️
config/src/helper.rs	46.00% <0.00%> (-4.00%)	⬇️
network-p2p/src/request_responses.rs	31.36% <0.00%> (-2.43%)	⬇️
executor/src/account.rs	41.09% <0.00%> (-1.51%)	⬇️
vm/types/src/token/token_value.rs	58.34% <0.00%> (-1.38%)	⬇️
... and 90 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 7ef1c62...16fedce. Read the comment docs.