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

internal/imgcache/cache.go

@@ -111,10 +111,16 @@ type Cache struct {
 	pendingAccessTimesDone    chan struct{}
 	pendingAccessTimesCancel  context.CancelFunc
 
-	// The counts of cache hits and misses, made and registered by
-	// registerCounters (see counters.go)
-	cacheHits   prometheus.Counter
-	cacheMisses prometheus.Counter
+	// The cache's metrics, made and registered by registerMetrics (see
+	// metrics.go)
+	cacheHits         prometheus.Counter
+	cacheMisses       prometheus.Counter
+	sourceCacheHits   prometheus.Counter
+	sourceCacheMisses prometheus.Counter
+	usageBytes        prometheus.Gauge
+	maxBytes          prometheus.Gauge
+	evictions         prometheus.Counter
+	evictedBytes      prometheus.Counter
 
 	// reconciliationPageSize is the most rows one read of a content table
 	// returns in the reconciliation pass. NewCache sets it to
@@ -128,7 +134,7 @@ type Cache struct {
 	reconciliationReadTestHook func(rows int)
 }
 
-// NewCache creates a new cache instance and registers its counters on
+// NewCache creates a new cache instance and registers its metrics on
 // metricsRegistry. pixad passes Prometheus' default registry, the one
 // /metrics serves. It makes no database call.
 func NewCache(
@@ -157,7 +163,7 @@ func NewCache(
 		reconciliationPageSize: defaultReconciliationPageSize,
 	}
 
-	err := c.registerCounters(metricsRegistry)
+	err := c.registerMetrics(metricsRegistry)
 	if err != nil {
 		return nil, err
 	}
@@ -656,17 +662,6 @@ func (c *Cache) CleanExpired(ctx context.Context) error {
 	}
 }
 
-// IncrementStats counts a cache hit or miss. It only adds to the cache's
-// Prometheus counters in memory (see counters.go), so it never waits for the
-// database or fails.
-func (c *Cache) IncrementStats(hit bool) {
-	if hit {
-		c.cacheHits.Inc()
-	} else {
-		c.cacheMisses.Inc()
-	}
-}
-
 // touchVariant hands the access time of a variant served from the cache to
 // the goroutine StartPendingAccessTimeWrites starts, which writes it in its
 // next batch. It never waits: while maxPendingAccessTimes access times of