feat(metrics): add relation busy rate to dashboard

[kwannoel]

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What's changed and what's your intention?

Provide relation busy rate. We take all the fragments of a relation and find the max busy rate amongst them. That will be the busy rate of the relation. We use topk so we can also provide the fragment label alongside the relation.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note

[kwannoel]

• feat(metrics): add relation busy rate to dashboard #23730 👈 (View in Graphite)
• feat(stream,dashboard): improve actor input buffer blocking metrics and add actor busy time panel #23612 : 1 other dependent PR (#23675 )
• main

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Pull Request Overview

This PR adds relation-level busy rate metrics to the Grafana dashboard by aggregating actor busy rates per materialized view. The change refactors existing actor busy rate logic into reusable functions and introduces new dashboard panels showing relation busy rates at multiple time intervals.

Key Changes

• Refactored _actor_busy_rate_alt_target into separate _actor_busy_rate_expr and _actor_busy_rate_target functions for code reuse
• Added _relation_busy_rate_expr and _relation_busy_rate_target functions to compute busy rates aggregated by relation
• Added four new dashboard panels displaying relation busy rates with different time windows ($__rate_interval, 10m, 5m, 3m)