parachain_candidate_backing_signed_statements_total metric

[kamilsa]

Referenced issues

Description of the Change

Possible Drawbacks

Checklist Before Opening a PR

Before you open a Pull Request (PR), please make sure you've completed the following steps and confirm by answering 'Yes' to each item:

1. Code is formatted: Have you run your code through clang-format to ensure it adheres to the project's coding standards? [Yes|No]
2. Code is documented: Have you added comments and documentation to your code according to the guidelines in the project's contributing guidelines? [Yes|No]
3. Self-review: Have you reviewed your own code to ensure it is free of typos, syntax errors, logical errors, and unresolved TODOs or FIXME without linking to an issue? [Yes|No]
4. Zombienet Tests: Have you ensured that the zombienet tests are passing? Zombienet is a network simulation and testing tool used in this project. It's important to ensure that these tests pass to maintain the stability and reliability of the project. [Yes|No]