Backport release/v6.7: fix(seidb): report only the current migration boundary on the snapshot gauge

[seidroid]

Backport of #4327 to release/v6.7.

[cursor]

PR Summary

Low Risk
Observability-only change to migration metrics emission; migration routing and persistence behavior are unchanged.

Overview
Fixes migration boundary Prometheus cardinality by replacing the periodic synchronous seidb_migration_boundary_snapshot gauge (which retained every historical boundary_hex label) with an OpenTelemetry Int64ObservableGauge that reports only the current boundary on each scrape.

NewMigrationMetrics no longer takes a snapshot interval; the 10s ticker loop and recordBoundarySnapshot writes are gone. A RegisterCallback observer reads in-memory boundary/version state, emits boundary_hex as the live boundary or complete at targetVersion, and Close cancels context so a small goroutine unregisters the callback. Router builders and comments are updated accordingly.

New tests assert a single label per collect, the complete sentinel at target version, and no export after Close.

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[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Sep 25, 2026, 7:58 PM

[seidroid]

This is a clean backport of #4327. It swaps the synchronous boundary-snapshot gauge and its ticker goroutine for an observable gauge with a registered callback, so each collection reports only the current boundary_hex label (or complete). The callback is unregistered when ctx is cancelled or Close is called. All three NewMigrationMetrics call sites are updated, and the new ManualReader tests cover the current-boundary, complete, and post-Close behavior. I found nothing blocking; I couldn't run the tests here because running them needed approval.

Findings: 0 blocking | 0 non-blocking | 0 posted inline

Blockers

• None at the file/PR level.

Non-blocking

• None at the file/PR level.

[codecov]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.54%. Comparing base (d756203) to head (23b340c).

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/migration/migration_metrics.go	85.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v6.7    #4348      +/-   ##
================================================
- Coverage         61.37%   60.54%   -0.84%     
================================================
  Files              2163     2081      -82     
  Lines            189085   179231    -9854     
================================================
- Hits             116059   108513    -7546     
+ Misses            62301    60786    -1515     
+ Partials          10725     9932     -793     

Flag	Coverage Δ
sei-db	69.80% <ø> (-0.22%)	⬇️
sei-db-state-db	?
sei-db-state-db-pr	79.12% <86.95%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/state_db/sc/composite/store.go	70.48% <ø> (ø)
sei-db/state_db/sc/migration/migration_manager.go	95.53% <ø> (ø)
sei-db/state_db/sc/migration/router_builder.go	57.71% <100.00%> (ø)
sei-db/state_db/sc/migration/migration_metrics.go	90.53% <85.00%> (+0.75%)	⬆️

... and 83 files with indirect coverage changes

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