[metrics] Rework on txpool metrics.

[jolestar]

1. 重构了 txpool 的 metrics， 通过 counter 统计 event 而不是 gauge。
2. 增加了测试 metrics 的方法，并在 txpool 测试中使用。

[codecov]

Codecov Report

Merging #2961 (5ad805d) into master (cd108db) will decrease coverage by 0.34%.
The diff coverage is 27.66%.

@@            Coverage Diff             @@
##           master    #2961      +/-   ##
==========================================
- Coverage   33.34%   33.01%   -0.33%     
==========================================
  Files         516      516              
  Lines       45141    45269     +128     
  Branches    20545    20623      +78     
==========================================
- Hits        15048    14941     -107     
- Misses      15896    16045     +149     
- Partials    14197    14283      +86     

Flag	Coverage Δ
unittests	33.01% <27.66%> (-0.33%)	⬇️

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

Impacted Files	Coverage Δ
commons/metrics/src/lib.rs	16.13% <ø> (+1.62%)	⬆️
test-helper/src/lib.rs	100.00% <ø> (ø)
txpool/src/lib.rs	44.74% <7.70%> (-2.58%)	⬇️
txpool/src/test.rs	15.16% <7.90%> (-6.12%)	⬇️
txpool/src/counters.rs	33.34% <33.34%> (-45.23%)	⬇️
txpool/src/tx_pool_service_impl.rs	46.24% <36.00%> (+2.24%)	⬆️
test-helper/src/txpool.rs	30.77% <37.50%> (+5.77%)	⬆️
rpc/server/src/module/pubsub/tests.rs	20.67% <40.00%> (+1.08%)	⬆️
config/src/metrics_config.rs	55.18% <47.83%> (-2.89%)	⬇️
vm/functional-tests/src/genesis_accounts.rs	37.50% <0.00%> (-25.00%)	⬇️
... and 94 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 cd108db...5ad805d. Read the comment docs.