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, in internal/imgcache/counters.go, on the default registry that
/metrics serves. The Increment methods add to them, 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 counters count for the whole process, so the tests that check how much
they rose do not call t.Parallel. Two stop tests whose write under way was a
hit count now use an access time.

Model: opus-5-5

internal/handlers/handlers.go

@@ -76,8 +76,8 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 		OnStart: func(_ context.Context) error {
 			return s.initImageService()
 		},
-		// The pending counts and access times are written here, before the
-		// database's stop hook closes it.
+		// The pending access times are written here, before the database's
+		// stop hook closes it.
 		OnStop: func(ctx context.Context) error {
 			if s.imgCache == nil {
 				return nil
@@ -85,7 +85,7 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 
 			return errors.Join(
 				s.imgCache.StopEviction(ctx),
-				s.imgCache.StopPendingCountWrites(ctx),
+				s.imgCache.StopPendingAccessTimeWrites(ctx),
 			)
 		},
 	})
@@ -128,9 +128,9 @@ func (s *Handlers) initImageService() error {
 	// rows, which is all it does when the disk cache is disabled.
 	cache.StartEviction(imgcache.DefaultEvictionInterval)
 
-	// Writes the counts and the access times of cache hits that requests
-	// hand over, so that no request waits for the database to record them
-	cache.StartPendingCountWrites()
+	// Writes the access times that requests hand over, so that no request
+	// waits for the database to record them
+	cache.StartPendingAccessTimeWrites()
 
 	// Create the fetcher config
 	fetcherCfg := httpfetcher.DefaultConfig()