Expose UFFD pager stats as Prometheus metrics#297
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Wire the pager's existing atomic counters into hypeman's OTel Prometheus bridge so otel-collector can scrape them alongside the other host exporters. The JSON /stats endpoint over the control unix socket is unchanged; a new opt-in TCP /metrics endpoint reads the same snapshot. --metrics-addr enables the pull endpoint. --otel-endpoint enables OTLP push. Both are opt-in so existing deployments keep working. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Pair with the new --metrics-addr flag: pass through an env var so per-instance overrides written via EnvironmentFile can opt in without editing the base unit. Empty default keeps the metrics server off for hosts that haven't been configured. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Required by scripts/check-uffd-version.sh whenever runtime files under lib/uffdpager change. The new metrics endpoint is additive and does not alter the UFFD wire protocol, but the pre-commit contract asks for an explicit version bump on any pager code change. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Summary
New flags on `hypeman-uffd-pager` (all opt-in, empty = disabled):
Metrics exposed
Counters: `hypeman_uffd_{cache_hits,cache_misses,faults,backing_bytes_read,copies,copy_errors}_total`, plus `_nanos_total` accumulators for cache lookup / add / fault / read-page / backing-read / copy latencies.
Gauges: `hypeman_uffd_{cache_bytes,cache_max_bytes,cache_items,cache_shards,active_sessions,active_faults,max_concurrent_faults,draining}`, plus high-water `_max_nanos` gauges for the same latencies.
Test plan