Count hits, misses, fetches and transforms as Prometheus counters (closes #311)

The five counts the cache_stats table held are now Prometheus counters in
memory, as fields of the cache. NewCache makes them and registers them on
the registry it is given. pixad gives it Prometheus' default registry,
which /metrics serves; each test gives it a new one. The Increment methods
add to their cache's counters, so counting makes no database call.

cache_stats, Cache.Stats, Service.Stats, CacheStats and the pending counts
are gone, with their tests. The background goroutine now writes only access
times, and its names say so: StartPendingAccessTimeWrites,
StopPendingAccessTimeWrites and pendingaccesstimes.go. The two stop tests
whose write under way was a count now use access times.

Model: opus-5-5

internal/imgcache/counters.go

@@ -0,0 +1,45 @@
+package imgcache
+
+import (
+	"fmt"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// registerCounters makes the cache's counters and registers them on
+// metricsRegistry. The counters are kept in memory, so adding to one waits
+// for nothing and makes no database call. It fails when metricsRegistry
+// already holds counters of the same names.
+func (c *Cache) registerCounters(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.upstreamFetches = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_upstream_fetches_total",
+		Help: "Fetches of a source image from upstream that read any bytes.",
+	})
+	c.upstreamFetchBytes = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_upstream_fetch_bytes_total",
+		Help: "Bytes read from upstream by fetches of source images.",
+	})
+	c.transforms = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_transforms_total",
+		Help: "Images the image processor transformed.",
+	})
+
+	for _, counter := range []prometheus.Counter{
+		c.cacheHits, c.cacheMisses, c.upstreamFetches, c.upstreamFetchBytes, c.transforms,
+	} {
+		err := metricsRegistry.Register(counter)
+		if err != nil {
+			return fmt.Errorf("failed to register the cache's counters: %w", err)
+		}
+	}
+
+	return nil
+}