feat(core): add metrics/observability with prometheus

[fborello-lambda]

Motivation

We need a starting point to create dashboards with Grafana. Metrics are essential to observe the behavior of the node.

Description

• Simple dashboard with a counter. Counts the transactions.

  • We neeed an API with the /metrics endpoint → using axum for fast development, we may want to create a simple API from scratch without any crate.

• To test it out:

1. cd crates/l2 && cp .env.example .env
2. make rm_db_l2 && make init

• In a new terminal:

1. make cli
2. ethrex_l2 test load --path <full_path>/ethrex/test_data/private_keys.txt -i 1

• In the browser go to http://localhost:3802/:
  • Login with admin/admin
  • Go to Dashboards → open the Ethrex - Transactions dashboard, the dashboard should show 172. The previous cli's command sent 172 transactions.

Example output:

Closes #1429