fix: stale gauge

[franklinkim]

Description

Make maestro.roster.players gauge observable instead of per-publish: fixes it going stale on idle soloist or resync-only rounds.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• 🏃‍➡️ Performance
• ✅ Tests
• 🔐 Security
• 🔧 Build/CI

Changes

• Convert maestro.roster.players from a per-Publish Int64Gauge to an observable Int64ObservableGauge that reads roster.Participants() on each metric collection
• Remove recordRoster() method; metric is now driven by callback instead of explicit recording
• Update metrics_test.go to verify gauge follows membership changes (join, leave, expiry) without explicit Publish
• Add coordinator_test.go acceptance test validating players report correctly after initial Publish with no subsequent ones

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.