Get PA single-stat value from the defined measure

What does this MR do and why?

Related to Get single-stat value from the defined measure (#385780 - closed)

For the single-stat query, we're getting the first "cell" (first column and row) from the raw results matrix. Regardless of the structure of the results.

This MR improves the precedence of the source of this value, pretty similar to how the Cube.js Playground's "Number" chart, in the following order:

1. The first defined measure in the Cube query.
2. The default Jitsu.count measure.
3. The first available cell.

Screenshots or screen recordings

No visual changes! 💯

How to set up and validate locally

Please follow the instructions at https://gitlab.com/gitlab-org/gitlab/-/snippets/2474959 to enable Product Analytics on your GDK.

With the Product Analytics feature enabled:

1. Go to /gitlab-org/gitlab-shell/-/product_analytics/dashboards/dashboard_overview.
2. The single-stat widget should render with a positive numeric value.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.