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
- 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/metrics.go

@@ -0,0 +1,88 @@
+package imgcache
+
+import (
+	"fmt"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// registerMetrics makes the cache's metrics and registers them on
+// metricsRegistry. The metrics are kept in memory, so updating one waits
+// for nothing and makes no database call. It fails when metricsRegistry
+// already holds metrics of the same names.
+func (c *Cache) registerMetrics(metricsRegistry prometheus.Registerer) error {
+	c.cacheHits = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_hits_total",
+		Help: "Image requests served from the cache.",
+	})
+	c.cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_misses_total",
+		Help: "Image requests not served from the cache.",
+	})
+	c.sourceCacheHits = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_source_cache_hits_total",
+		Help: "Variants made or checked whose source was in the cache.",
+	})
+	c.sourceCacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_source_cache_misses_total",
+		Help: "Variants made or checked whose source was not in the cache.",
+	})
+	c.usageBytes = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "pixa_cache_usage_bytes",
+		Help: "Bytes of sources and variants in the cache, as eviction last read them.",
+	})
+	c.maxBytes = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "pixa_cache_max_bytes",
+		Help: "The cache size limit in bytes, cache_max_bytes.",
+	})
+	c.evictions = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_evictions_total",
+		Help: "Sources and variants removed by eviction.",
+	})
+	c.evictedBytes = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_evicted_bytes_total",
+		Help: "Bytes of the sources and variants removed by eviction.",
+	})
+
+	c.maxBytes.Set(float64(c.config.MaxBytes))
+
+	for _, metric := range []prometheus.Collector{
+		c.cacheHits, c.cacheMisses, c.sourceCacheHits, c.sourceCacheMisses,
+		c.usageBytes, c.maxBytes, c.evictions, c.evictedBytes,
+	} {
+		err := metricsRegistry.Register(metric)
+		if err != nil {
+			return fmt.Errorf("failed to register the cache's metrics: %w", err)
+		}
+	}
+
+	return nil
+}
+
+// IncrementStats counts a cache hit or miss of an image request. It only
+// adds to the cache's counters in memory, so it never waits for the
+// database or fails.
+func (c *Cache) IncrementStats(hit bool) {
+	if hit {
+		c.cacheHits.Inc()
+	} else {
+		c.cacheMisses.Inc()
+	}
+}
+
+// IncrementSourceStats counts whether a variant being made or checked found
+// its source in the cache. Like IncrementStats, it never waits or fails.
+func (c *Cache) IncrementSourceStats(hit bool) {
+	if hit {
+		c.sourceCacheHits.Inc()
+	} else {
+		c.sourceCacheMisses.Inc()
+	}
+}
+
+// countEviction counts one source or variant of sizeBytes bytes removed by
+// eviction.
+func (c *Cache) countEviction(sizeBytes int64) {
+	c.evictions.Inc()
+	c.evictedBytes.Add(float64(sizeBytes))
+}