refactor: replay metrics come from spans, and ReplayMetrics is gone

[HardMax71]

Summary

Part of #389, the replay area, and fixes #318: the replay queue size gauge carried a session_id label, so every session left its own series behind. The fifteen instruments in ReplayMetrics now come from spans.

Changes

EventReplayService leaves a span where it used to record each metric: replay.session_created, replay.session_started, replay.status_change, replay.event, replay.batch, replay.session_completed and replay.target. The collector's signaltometrics connector turns them into the same series. The API's scheduler gets one more job: every 10 s report_sessions reports the sessions held per status, zeros included, and the loaded events still to send, summed over sessions. replay_sessions_active goes, since running plus paused from the per-status gauge is the same number, and the dashboard panel now sums those. Labels take the span attribute names (replay_status, replay_target, ...).

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or tooling changes

Test Plan

• Backend unit tests pass (cd backend && uv run pytest tests/unit)
• Frontend unit tests pass (cd frontend && npm run test)
• Backend E2E tests pass (docker compose exec -T backend sh -c 'uv sync --group test --no-dev --frozen --no-install-project --no-build && pytest tests/e2e')
• Frontend E2E tests pass (cd frontend && npm run test:e2e)
• API spec and client regenerated if routes or schemas changed (./deploy.sh openapi, then ./deploy.sh types), never edited by hand
• Manual testing performed

Validation: unit 570 passed, contract 3 passed, e2e 488 passed and 1 skipped. Four real sessions run from the API container gave exactly the expected totals at the collector (4 created, 4 batches of 10, speed multipliers summing to 398). Duration and throughput are only unit-tested: no live session could complete, since main's dispatch job skips its own next run when the delay is short.

Checklist

• My code follows the project's style guidelines
• I have run pre-commit run --all-files and all checks pass
• I have added tests that prove my fix/feature works
• I have updated documentation if needed
• My changes don't introduce new warnings

---

Summary by cubic

Moves replay metrics from the ReplayMetrics instruments to spans that the collector's signaltometrics connector turns into the same series, deleting ReplayMetrics as part of #389. Fixes #318 by dropping the session_id label from the queue size gauge, which had created a series per session.

• EventReplayService now records replay.session_created, replay.session_started, replay.status_change, replay.event, replay.batch, replay.session_completed, and replay.target spans.
• The API scheduler runs report_sessions every 10 s to report sessions per status (zeros included) and loaded events still to send.
• replay_sessions_active is gone; the dashboard panel sums running and paused from the per-status gauge instead.
• Labels use the span attribute names (replay_status, replay_target, ...).

^{Written for commit a56db99. Summary will update on new commits.}

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[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ackend/app/services/event_replay/replay_service.py	97.43%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Flag	Coverage Δ
backend-e2e	82.77% <86.95%> (-0.14%)	⬇️
backend-unit	73.88% <69.56%> (+0.66%)	⬆️

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

Files with missing lines	Coverage Δ
backend/app/core/dishka_lifespan.py	100.00% <100.00%> (ø)
backend/app/core/metrics/__init__.py	100.00% <ø> (ø)
backend/app/core/providers.py	95.43% <ø> (-0.06%)	⬇️
...ackend/app/services/event_replay/replay_service.py	85.66% <97.43%> (+4.06%)	⬆️

... and 2 files with indirect coverage changes

Components	Coverage Δ
Backend	92.15% <97.82%> (+0.03%)	⬆️
Frontend	92.60% <ø> (ø)

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