DBMON-4906 - Update Postgres dashboard aggregation functions for postgresql.connections

[Yoone]

What does this PR do?

It updates the 3 postgresql.connections queries on the Postgres Overview and Postgres Metrics dashboards to use sum instead of avg/max, and adds .weighted() for time-averaged values. The "Active connections" line keeps .rollup(max) to preserve the peak-vs-max_connections comparison.

Motivation

Ensure that the out of the box dashboards are consistent with DBM use of the postgresql.connections metric.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[dd-octo-sts]

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