feat(telemetry): export admission_control.limit metric

[alco]

Summary

• Moves limit from event metadata into measurements in Electric.AdmissionControl for both [:electric, :admission_control, :acquire] and [:electric, :admission_control, :reject] events.
• Exports two new last_value metrics tagged by :kind in StackTelemetry:
  • electric.admission_control.acquire.limit
  • electric.admission_control.reject.limit
• With the limit now exported alongside acquire.current, the "Electric — Admission Control" dashboard (stratovolt#1467) can plot fill percentage = acquire.current / acquire.limit per kind.

Closes electric-sql/stratovolt#1471

Test plan

• mix test in packages/electric-telemetry (99 tests pass)
• mix test test/electric/admission_control_test.exs in packages/sync-service (11 tests pass)
• Verify electric.admission_control.acquire.limit and electric.admission_control.reject.limit columns appear in the admin-electric / electric-region datasets on Honeycomb after deploy

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.37%. Comparing base (388ec63) to head (e163f8f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4147      +/-   ##
==========================================
- Coverage   89.20%   85.37%   -3.84%     
==========================================
  Files          25       40      +15     
  Lines        2520     3077     +557     
  Branches      636      641       +5     
==========================================
+ Hits         2248     2627     +379     
- Misses        270      448     +178     
  Partials        2        2              

Flag	Coverage Δ
electric-telemetry	68.04% <ø> (?)
elixir	68.04% <ø> (?)
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (ø)
packages/start	82.83% <ø> (ø)
packages/typescript-client	94.30% <ø> (ø)
packages/y-electric	56.05% <ø> (ø)
typescript	89.20% <ø> (ø)
unit-tests	85.37% <ø> (-3.84%)	⬇️

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[claude]

Claude Code Review

Summary

This iteration moves limit from event metadata into measurements for both the acquire and reject telemetry events, and adds electric.admission_control.reject.limit alongside the previously added acquire.limit. The structural change is correct and directly resolves the defensive metadata-access concern raised in iteration 2.

What's Working Well

• Moving limit to measurements is the right model: it's a quantity, not context, and it aligns with how current is already handled in the acquire event.
• reject.limit as a last_value tagged by :kind is a natural peer to acquire.limit and enables the same fill-percentage calculation for the reject path.
• Tests updated correctly — assertions now reference measurements.limit instead of metadata.limit for both events.
• Changeset covers both electric-telemetry and sync-service.

Issues Found

Suggestions (Nice to Have)

No tests for the new metric definitions

Neither electric.admission_control.acquire.limit nor the new electric.admission_control.reject.limit has a test in the electric-telemetry package verifying the metric is wired up and correctly extracts the measurement. The sync-service tests confirm the telemetry event shape is correct, but a telemetry package test would catch regressions if the metric name or extraction logic drifts. Low risk given the simplicity, but worth adding alongside the existing measurement_test.exs pattern.

Issue Conformance

No linked issue is directly accessible (private repo), but the PR description clearly explains the motivation (fill-percentage dashboards, stratovolt#1471) and the implementation matches it completely.

Previous Review Status

• Metric renamed acquire.limit ✓ (iteration 2)
• Defensive Map.get(metadata, :limit) suggestion → RESOLVED: limit is now in measurements; the default Telemetry.Metrics key extraction is used (no custom measurement function), so a missing :limit is handled silently at the reporter layer rather than raising a KeyError.
• Missing test for acquire.limit → still open; now also applies to reject.limit.

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Review iteration: 3 | 2026-04-21