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/imgcache/cache_hit_internal_test.go

@@ -167,8 +167,8 @@ func TestService_Get_MakesAgainVariantOfNoOutputFormat(t *testing.T) {
 
 // TestService_Get_HitWaitsForNoDatabaseWrite holds the database's one
 // connection while a cached variant is requested. The hit must be answered
-// while the connection is still held, and its access time and hit count
-// must reach the database once it is free.
+// while the connection is still held, and its access time must reach the
+// database once it is free.
 func TestService_Get_HitWaitsForNoDatabaseWrite(t *testing.T) {
 	t.Parallel()
 
@@ -177,14 +177,18 @@ func TestService_Get_HitWaitsForNoDatabaseWrite(t *testing.T) {
 	key := imagerequest.CacheKey(req)
 
 	getImage(t, svc, req)
-	waitForPendingCountsWritten(t, svc.cache)
+
+	// Each connection to an in-memory SQLite database opens a new, empty one,
+	// so the goroutine that writes the access times must share the
+	// connection that holds the schema.
+	svc.cache.db.SetMaxOpenConns(1)
+	svc.cache.StartPendingAccessTimeWrites()
+
+	defer func() { _ = svc.cache.StopPendingAccessTimeWrites(t.Context()) }()
 
 	anHourAgo := time.Now().Add(-time.Hour)
 	setVariantLastAccessed(t, svc.cache, key, anHourAgo)
 
-	want := readCacheStatsCounters(t, svc.cache)
-	want.hitCount++
-
 	releaseDatabase := holdDatabase(t, svc.cache)
 	defer releaseDatabase()
 
@@ -195,15 +199,13 @@ func TestService_Get_HitWaitsForNoDatabaseWrite(t *testing.T) {
 		if got.err != nil {
 			t.Fatalf("Get() of the cached variant error = %v", got.err)
 		}
-	case <-time.After(pendingCountsWait):
+	case <-time.After(pendingAccessTimesWait):
 		t.Fatal("the cache hit waited for the database")
 	}
 
 	releaseDatabase()
 
-	waitForCounters(t, svc.cache, want)
-
-	deadline := time.Now().Add(pendingCountsWait)
+	deadline := time.Now().Add(pendingAccessTimesWait)
 
 	for countRows(t, svc.cache, `
 		SELECT COUNT(*) FROM variant_content
@@ -239,7 +241,7 @@ func TestTouchVariantAndTouchSourceContent_DropAccessTimesPastTheLimit(t *testin
 
 	select {
 	case <-done:
-	case <-time.After(pendingCountsWait):
+	case <-time.After(pendingAccessTimesWait):
 		t.Fatal("touchVariant or touchSourceContent waited once the access " +
 			"times reached the limit")
 	}
@@ -255,11 +257,11 @@ func TestTouchVariantAndTouchSourceContent_DropAccessTimesPastTheLimit(t *testin
 	}
 }
 
-// TestStopPendingCountWrites_WritesPendingAccessTimes hands the access time
-// of one of two variants, three times, then stops the goroutine that writes
-// them, which never started. The stop must write that access time and
+// TestStopPendingAccessTimeWrites_WritesPendingAccessTimes hands the access
+// time of one of two variants, three times, then stops the goroutine that
+// writes them, which never started. The stop must write that access time and
 // leave the other variant's.
-func TestStopPendingCountWrites_WritesPendingAccessTimes(t *testing.T) {
+func TestStopPendingAccessTimeWrites_WritesPendingAccessTimes(t *testing.T) {
 	t.Parallel()
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
@@ -274,9 +276,9 @@ func TestStopPendingCountWrites_WritesPendingAccessTimes(t *testing.T) {
 		cache.touchVariant(testVariantKeyOne)
 	}
 
-	err := cache.StopPendingCountWrites(t.Context())
+	err := cache.StopPendingAccessTimeWrites(t.Context())
 	if err != nil {
-		t.Fatalf("StopPendingCountWrites() error = %v", err)
+		t.Fatalf("StopPendingAccessTimeWrites() error = %v", err)
 	}
 
 	for key, wantTouched := range map[imagerequest.VariantKey]int{