fix(metrics): label lock wait session pids

Summary

Closes #199 (closed).

Promote lock_waits session pids from numeric metric values to Prometheus labels by emitting them as tag_blocked_pid and tag_blocker_pid text columns. This disambiguates concurrent same-table lock waits that otherwise share the same non-pid label set and collide as duplicate Prometheus series.

Verification

python3 -m pytest -q tests/compliance_vectors/test_work_item_199_lock_waits.py
# 1 passed

Notes

No local product/PostgreSQL stack was started for this release fix. This is a targeted static/config fix for the metric shape described in #199 (closed).