Export cache, eviction, upstream and transform metrics (closes #85)

New pixa_ metrics, none with labels:
- source cache hits and misses, counted once per variant made or checked;
  a source whose cached file cannot be used is a miss
- 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

configs/config.example.yml

@@ -158,8 +158,9 @@ access_control_allow_origin: "*"
 # Sentry DSN for error reporting (default: empty, which turns it off)
 sentry_dsn: ""
 
-# Username and password for /metrics, set together (default: unset). Metrics
-# are measured and /metrics is served only when both are set.
+# Username and password for /metrics, set together (default: unset). pixa
+# serves /metrics, and measures HTTP requests, only when both are set. It
+# measures the pixa_ metrics either way.
 # metrics:
 #   username: "admin"
 #   password: "secret"