When a gunicorn worker exits (e.g. from max_requests recycling), its
per-process prometheus .db files remain on disk. For gauges using
livesum/liveall mode, this means the dead worker's last-known values
persist as if the process were still alive. Wire gunicorn's child_exit
hook to call mark_process_dead() so live-tracking gauges accurately
reflect only running workers.
Phase 2 (per-worker mark_process_dead on shutdown) only ever fired
when all workers shut down together, making it redundant — Phase 1
wipes everything on next startup anyway. This aligns with the
prometheus_client docs: just wipe the directory between runs.
Adds cleanup utilities for PROMETHEUS_MULTIPROC_DIR to prevent
unbounded RAM/disk growth from stale .db files in multi-worker setups.
Three-part lifecycle aligned with upstream prometheus_client docs:
1. Startup: wipe entire directory before workers fork (clean slate)
2. Shutdown: mark_process_dead() for own PID (removes gauge_live* only)
3. Periodic (hourly): scan for dead PIDs and call mark_process_dead()
Counter/histogram files are never individually deleted at runtime to
avoid partial counter resets that cause false spikes in rate()/increase().
Also auto-creates PROMETHEUS_MULTIPROC_DIR when prometheus callback is
configured with multiple workers and the env var is not already set.
