feat: add p2p metrics

[varex83]

This PR implements P2P metrics, which are fully compatible with Go's implementation.

To verify that it's correct:

1. Run local grafana & prometheus (by simply docker-compose up in /test-infra dir)
2. Run the example in Rust cargo run --example metrics -p charon-p2p
3. Create file (p2p/metrics_example_test.go) in Go's implementation with this contents
4. And run go's example with go test -run TestMetricsServer -timeout 20s ./p2p

Then to verify: go to grafana and make sure all metrics exist with the same names and labels, the implementation names could be found in peer field (go and rust accordingly).

Closes #93

[github-actions]

Coverage (base → head): 61.23% → 61.03% ⬇️ 0.2 pp
Only posts when PR coverage is lower than the base branch's latest push (or freshly computed base).

[emlautarom1]

LGTM, I don't see anything particularly problematic.