Feat: Multiple Metrics in Leaderboards

[lovincyrus]

This pull request introduces support for multiple metrics in leaderboards, enabling users to adjust the leaderboard_measure_count to control the number of visible measures. This featured is gated by feature flag leaderboardMeasureCount

features:
  - leaderboardMeasureCount

Closes #6713

https://www.notion.so/rilldata/Multiple-Metrics-in-Leaderboards-19aba33c8f57802f98aeefe569a81b39

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[ericpgreen2]

Overall, the code looks like it's on track! I played around with the branch and observed bugs and opportunity for UX polish, but I won't enumerate all those given the PR is in draft state and I assume you see them too. I'll just say that the most jarring IMO is the layout shift when toggling the leaderboard's sorted measure.