Adds four metrics surfaced via the existing /metrics endpoint:
- overpass_cache_events_total{result=hit|miss|coalesced}
- overpass_cache_size (gauge)
- overpass_upstream_duration_seconds (histogram)
- overpass_upstream_requests_total{status} — covers 2xx/4xx/5xx and
an explicit "error" label for network-level failures
Grafana dashboards/planner.json gets a new row:
- Upstream health (5m 2xx success ratio, red <90%, green >99%)
- Cache hit ratio
- Cache size
- Upstream p95 latency
Plus timeseries panels for cache event breakdown, upstream status
over time, and p50/p95/p99 upstream latency.
The success-rate formula uses clamp_min(..., 1) on the denominator so
it doesn't NaN when traffic is zero.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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