Add metrics for mempool change

[yzang2019]

Describe your changes and provide context

This PR added two new metrics for tendermint mempool, which tracks number of txs inserted and removed from mempool

Testing performed to validate your change

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 25.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 57.88%. Comparing base (3affbcf) to head (7c60726).

❗ Current head 7c60726 differs from pull request most recent head 39787ff

Please upload reports for the commit 39787ff to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   58.19%   57.88%   -0.32%     
==========================================
  Files         249      249              
  Lines       34445    34022     -423     
==========================================
- Hits        20045    19693     -352     
+ Misses      12814    12754      -60     
+ Partials     1586     1575      -11     

Files	Coverage Δ
internal/mempool/mempool.go	70.16% <100.00%> (-2.72%)	⬇️
internal/mempool/metrics.gen.go	16.17% <14.28%> (+0.32%)	⬆️

... and 18 files with indirect coverage changes