Add a new metric on backends count and status for proxysql

[aymeric-ledizes]

What does this PR do?

This PR add a new additional metric to know the numbers of mysql backends and their status (as tags of the metric)

Motivation

Currently there is a service check on backend status but no metrics.
The service check does not meet our needs as it does not allow to customize queries (ex: no custom time range evaluation)

Additional Notes

Despite there is already a service check proxysql.backend.status. This PR seems the only way to correctly monitor and alert on mysql backends count/status

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #11812 (6446915) into master (70701cd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Flag	Coverage Δ
proxysql	98.97% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[aymeric-ledizes]

Thanks for the approve @maycmlee 🙏 Can we merge this PR?

[maycmlee]

Hi @aymeric-ledizes! Thanks for contributing. This is waiting on an engineering review before it can be merged.