Etherlink/Metrics: alerts for kernel governance

Context

Having alerts on new proposal/votings/upgrades/period type changes could be relevant to monitor the kernel governance contract more easily.

The alerts can't be imported automatically and require some manual tweaking to the file, I followed what's written on etherlink/scripts/grafana/Alert_README.md to have some consistency across the different alerts that we submit to the tezos repo.

Manually testing the MR

Use and tweak the second commit to have a small scenario locally.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR