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/database/concurrent_writes_internal_test.go

@@ -104,33 +104,24 @@ func writeConcurrently(t *testing.T, db *sql.DB) {
 
 	wg.Wait()
 
-	var hits, sources int
+	var sources int
 
-	err := db.QueryRowContext(ctx, `
-		SELECT hit_count, (SELECT COUNT(*) FROM source_content)
-		FROM cache_stats WHERE id = 1
-	`).Scan(&hits, &sources)
+	err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM source_content`).
+		Scan(&sources)
 	if err != nil {
 		t.Fatalf("failed to count the writes: %v", err)
 	}
 
-	if hits != totalRequests || sources != totalRequests {
-		t.Errorf("hit_count = %d and %d source_content rows, want %d of each",
-			hits, sources, totalRequests)
+	if sources != totalRequests {
+		t.Errorf("%d source_content rows, want %d", sources, totalRequests)
 	}
 }
 
 // writeLikeOneRequest makes the writes one request and the eviction pass
-// make: it counts a cache hit, stores a source, records a transformed image
-// and deletes that record again.
+// make: it stores a source, records a transformed image and deletes that
+// record again.
 func writeLikeOneRequest(ctx context.Context, db *sql.DB, key string) error {
-	_, err := db.ExecContext(ctx,
-		`UPDATE cache_stats SET hit_count = hit_count + 1 WHERE id = 1`)
-	if err != nil {
-		return fmt.Errorf("counting a cache hit: %w", err)
-	}
-
-	_, err = db.ExecContext(ctx, `INSERT INTO source_content
+	_, err := db.ExecContext(ctx, `INSERT INTO source_content
 		(source_key, size_bytes) VALUES (?, 1)`, key)
 	if err != nil {
 		return fmt.Errorf("storing a source: %w", err)

internal/database/database_internal_test.go

@@ -159,7 +159,7 @@ func TestApplyMigrations_CreatesSchemaAndTables(t *testing.T) {
 	}
 
 	// Verify that the application tables created by 001.sql exist.
-	tables := []string{"source_content", "negative_cache", "cache_stats"}
+	tables := []string{"source_content", "negative_cache"}
 	for _, table := range tables {
 		var count int