New pixa_ metrics, none with labels:
- source cache hits and misses, counted once per variant made or checked
- cache usage and limit gauges; usage is set each time eviction reads it
- eviction count and bytes, from the size_bytes the deleted row returns
- upstream fetch duration histogram and fetch error counter
- transform duration histogram and an in-progress gauge, both including
the wait for a processing slot
Removing a damaged file during reconciliation is not counted as an
eviction. README lists every metric; the metrics config comment now says
the pixa_ metrics are measured whether or not /metrics is served.
Model: opus-5-5
