go/p2p: collect Prometheus metrics

[ptrus]

The issue called for adding a metric for number of blocked p2p peers specifically. Decided to add some other useful P2P metrics as well.

If there's any other P2P metrics that would be nice to collect, feel free to suggest.

Testing

Tested locally with E2E tests and metrics enabled:

[codecov]

Codecov Report

Merging #5327 (1f8bd34) into master (9551d85) will decrease coverage by 0.02%.
The diff coverage is 37.93%.

❗ Current head 1f8bd34 differs from pull request most recent head fdb3d92. Consider uploading reports for the commit fdb3d92 to get more accurate results

@@            Coverage Diff             @@
##           master    #5327      +/-   ##
==========================================
- Coverage   66.96%   66.95%   -0.02%     
==========================================
  Files         524      525       +1     
  Lines       55781    55809      +28     
==========================================
+ Hits        37355    37368      +13     
- Misses      13875    13897      +22     
+ Partials     4551     4544       -7     

Impacted Files	Coverage Δ
go/p2p/metrics.go	18.18% <18.18%> (ø)
go/p2p/p2p.go	72.59% <100.00%> (+0.62%)	⬆️

... and 54 files with indirect coverage changes