Add PG server version as tag

[bonnefoa]

What does this PR do?

Add a new postgresql_version tag to all metrics.

Motivation

Making version available as a metric tag provides multiple benefits:

• We can observe the change of version from minor or major upgrades that could explain instance restarts during a maintenance windows.
• We can list the number of instances per version and track outdated clusters

Additional Notes

copy.tag was removed as self.gauge is already creating a copy of normalised tags so there's no need for this additional copy.

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] Changelog entries must be created for modifications to shipped code
• [ ] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.