Add metrics for total txs bytes in mempool

[yzang2019]

Describe your changes and provide context

The current problem is that we are blind about what's the daily mempool total txs size in bytes, and we are not sure what would be a safe config value to set to limit the size of txs in mempool.

This PR address this concern by adding visibility to monitor total txs size in bytes in mempool.

Testing performed to validate your change

[codecov]

Codecov Report

Attention: Patch coverage is 76.74419% with 10 lines in your changes missing coverage. Please review.

Project coverage is 58.29%. Comparing base (fbf3097) to head (8fbeecc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   58.24%   58.29%   +0.04%     
==========================================
  Files         249      249              
  Lines       34450    34468      +18     
==========================================
+ Hits        20067    20094      +27     
+ Misses      12800    12790      -10     
- Partials     1583     1584       +1     

Files	Coverage Δ
internal/mempool/mempool.go	73.42% <91.66%> (+0.54%)	⬆️
internal/mempool/metrics.gen.go	16.09% <57.89%> (+0.23%)	⬆️

... and 17 files with indirect coverage changes