task(PM-22100): Add per-query Prometheus metrics for DBSync queries

[m2ux]

Summary

Enable Prometheus per-query timing histograms for midnight-node data sources by updating the local observed_async_trait! macro to record metrics.

🎫 Ticket 📐 Engineering

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Motivation

DBSync queries to Cardano mainnet are significantly slower than preview (~6s vs ~0.2s). The team needs per-query Prometheus timing histograms to identify which specific queries exceed block production time. Partner-chains data sources already have these metrics, but midnight-node data sources (cNight observation, federated authority, candidates) use a local macro that only logs — no Prometheus timing.

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Changes

Update: After investigation, no changes are needed in partner-chains. The McFollowerMetrics accessor methods are already pub and the observed_async_trait! macro is already #[macro_export]ed. All changes go in midnight-node — see the midnight-node PR.

This PR may be closed in favor of the midnight-node PR.

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📌 Submission Checklist

• Changes are backward-compatible (or flagged if breaking)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason: No changes in partner-chains
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• No new todos introduced

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🔱 Fork Strategy

• Node Runtime Update
• Node Client Update
• Other
• N/A

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🗹 TODO before merging

• Determine if this PR should be closed (no partner-chains changes needed)