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/cache_internal_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	_ "modernc.org/sqlite"
 )
 
@@ -33,16 +34,6 @@ func setupTestDB(t *testing.T) *sql.DB {
 			expires_at DATETIME NOT NULL,
 			UNIQUE(source_host, source_path, source_query)
 		);
-		CREATE TABLE cache_stats (
-			id INTEGER PRIMARY KEY CHECK (id = 1),
-			hit_count INTEGER NOT NULL DEFAULT 0,
-			miss_count INTEGER NOT NULL DEFAULT 0,
-			upstream_fetch_count INTEGER NOT NULL DEFAULT 0,
-			upstream_fetch_bytes INTEGER NOT NULL DEFAULT 0,
-			transform_count INTEGER NOT NULL DEFAULT 0,
-			last_updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
-		);
-		INSERT INTO cache_stats (id) VALUES (1);
 	`
 
 	_, err = db.ExecContext(t.Context(), schema)
@@ -53,48 +44,6 @@ func setupTestDB(t *testing.T) *sql.DB {
 	return db
 }
 
-func setupTestCache(t *testing.T) *Cache {
-	t.Helper()
-
-	tmpDir := t.TempDir()
-	db := setupTestDB(t)
-
-	cache, err := NewCache(db, CacheConfig{
-		StateDir:    tmpDir,
-		NegativeTTL: 5 * time.Minute,
-	})
-	if err != nil {
-		t.Fatalf("failed to create cache: %v", err)
-	}
-
-	return cache
-}
-
-func TestCache_Stats(t *testing.T) {
-	t.Parallel()
-
-	cache := setupTestCache(t)
-	ctx := context.Background()
-
-	// Increment some stats
-	cache.IncrementStats(true, 0)
-	cache.IncrementStats(true, 0)
-	cache.IncrementStats(false, 1024)
-
-	stats, err := cache.Stats(ctx)
-	if err != nil {
-		t.Fatalf("Stats() error = %v", err)
-	}
-
-	if stats.HitCount != 2 {
-		t.Errorf("Stats() HitCount = %d, want 2", stats.HitCount)
-	}
-
-	if stats.MissCount != 1 {
-		t.Errorf("Stats() MissCount = %d, want 1", stats.MissCount)
-	}
-}
-
 func TestCache_CleanExpired(t *testing.T) {
 	t.Parallel()
 
@@ -104,7 +53,7 @@ func TestCache_CleanExpired(t *testing.T) {
 	cache, _ := NewCache(db, CacheConfig{
 		StateDir:    tmpDir,
 		NegativeTTL: 1 * time.Millisecond,
-	})
+	}, prometheus.NewRegistry())
 
 	ctx := context.Background()
 
@@ -155,7 +104,7 @@ func TestCache_StorageDirectoriesCreated(t *testing.T) {
 
 	_, err := NewCache(db, CacheConfig{
 		StateDir: tmpDir,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("NewCache() error = %v", err)
 	}