Show the merged MR count on the DAP Impact MR cycle time panels

What does this MR do and why?

Adds a live merged merge request count above the two bars (Median and p75) in the DAP Impact dashboard's Work tab, MR cycle time section.

• Both the "Created by Duo" and "Not created by Duo" panels now select throughputCount.
• The description quotes it: "%{throughputCount} merged merge requests" for the Duo panel, with , baseline appended for the comparison panel.
• Since the description quotes throughputCount, it renders as text, not a third bar.

This is stacked on !256879 (merged), which adds the dynamic description support this relies on. Targets that MR's branch, not master, and should merge after it.

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/628932
• !256879 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the dap_impact_v1 feature flag.
2. Go to Explore > Analytics dashboards, open the DAP Impact dashboard, and switch to the Work tab.
3. In the MR cycle time section, check both panels show a merged merge request count above their bars, and still show exactly two bars (Median and p75).

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.