refactor(observability): self-migrate to register_or_reuse helpers (Phase 1)

[pcalnon]

Summary

Phase 1 of the migration plan in notes/observability/REGISTER_OR_REUSE_HELPER_DESIGN_2026-05-05.md.

juniper-observability's own two inline guards (added in PR #211 for PrometheusMiddleware.__init__ and PR #214 for set_build_info) swap to the canonical helpers introduced in 0.2.0 (PR #216 / tag juniper-observability-v0.2.0).

What changes

File	Helper used	Net diff
juniper_observability/middleware/prometheus.py	register_or_reuse (×3)	−25 lines, −1 closure, −1 REGISTRY import
juniper_observability/prometheus.py	register_info_or_update	−10 lines, −1 try/except, −1 REGISTRY import

Pure internal refactor — production behaviour and the collectors' wire format are unchanged. The helper has its own unit tests (tests/test_prometheus_helpers.py, 26 cases including the adopt-existing path); these two consumers' integration tests (test_prometheus_middleware.py, test_prometheus_utils.py) exercise the swap end-to-end.

Verification

• juniper-observability own suite: 83 passed (unchanged)
• juniper-data full default suite under JuniperData env: 950 passed (unchanged)

What's next

Phase 2 — per-consumer migration in dependency order: juniper-data → juniper-canopy → juniper-cascor-client → juniper-cascor. Each consumer's PR will:

1. Bump the juniper-observability dep pin to >=0.2.0
2. Replace its inline _get_or_create / _register_or_reuse with an import from juniper_observability
3. Drop the duplicated implementation

Test plan

• juniper-observability own tests pass (83 / 83)
• juniper-data tests pass (950 / 950) — exercises the helper through the integrated middleware + build-info paths
• CHANGELOG Unreleased entry under Changed documents the refactor
• No version bump in this PR (no public API change); whoever cuts the next release decides if 0.2.1 is warranted

🤖 Generated with Claude Code

[pcalnon]

approved